UnicodeDecodeError when import sas7bdat with special characters in Chinese variables
#### Code Sample, a copy-pastable example if possible

here is the sas7bdat files for test (Chinese names end with special characters),

[issue.zip](https://github.com/pandas-dev/pandas/files/1836642/issue.zip)

issue.sas7bdat has Chinese values, can be correctly imported by pandas.

![image](https://user-images.githubusercontent.com/3222056/37758460-40263ffc-2deb-11e8-92b4-3359afa21896.png)

issue1.sas7bdat has Chinese variables

![image](https://user-images.githubusercontent.com/3222056/37759053-6ea711e2-2ded-11e8-8057-44e58ab91931.png)

```python
# Your code here
df1=pd.read_sas('issue1.sas7bdat',encoding='GBK')
```

#### Problem description
```
---------------------------------------------------------------------------
UnicodeDecodeError                        Traceback (most recent call last)
<ipython-input-2-bcd7a0b4819c> in <module>()
----> 1 df1=pd.read_sas('issue1.sas7bdat',encoding='GBK')

~/.pyenv/versions/3.6.4/envs/ts/lib/python3.6/site-packages/pandas/io/sas/sasreader.py in read_sas(filepath_or_buffer, format, index, encoding, chunksize, iterator)
     59         reader = SAS7BDATReader(filepath_or_buffer, index=index,
     60                                 encoding=encoding,
---> 61                                 chunksize=chunksize)
     62     else:
     63         raise ValueError('unknown SAS format')

~/.pyenv/versions/3.6.4/envs/ts/lib/python3.6/site-packages/pandas/io/sas/sas7bdat.py in __init__(self, path_or_buf, index, convert_dates, blank_missing, chunksize, encoding, convert_text, convert_header_text)
     96 
     97         self._get_properties()
---> 98         self._parse_metadata()
     99 
    100     def close(self):

~/.pyenv/versions/3.6.4/envs/ts/lib/python3.6/site-packages/pandas/io/sas/sas7bdat.py in _parse_metadata(self)
    276                 raise ValueError(
    277                     "Failed to read a meta data page from the SAS file.")
--> 278             done = self._process_page_meta()
    279 
    280     def _process_page_meta(self):

~/.pyenv/versions/3.6.4/envs/ts/lib/python3.6/site-packages/pandas/io/sas/sas7bdat.py in _process_page_meta(self)
    282         pt = [const.page_meta_type, const.page_amd_type] + const.page_mix_types
    283         if self._current_page_type in pt:
--> 284             self._process_page_metadata()
    285         return ((self._current_page_type in [256] + const.page_mix_types) or
    286                 (self._current_page_data_subheader_pointers is not None))

~/.pyenv/versions/3.6.4/envs/ts/lib/python3.6/site-packages/pandas/io/sas/sas7bdat.py in _process_page_metadata(self)
    312                 self._get_subheader_index(subheader_signature,
    313                                           pointer.compression, pointer.ptype))
--> 314             self._process_subheader(subheader_index, pointer)
    315 
    316     def _get_subheader_index(self, signature, compression, ptype):

~/.pyenv/versions/3.6.4/envs/ts/lib/python3.6/site-packages/pandas/io/sas/sas7bdat.py in _process_subheader(self, subheader_index, pointer)
    382             raise ValueError("unknown subheader index")
    383 
--> 384         processor(offset, length)
    385 
    386     def _process_rowsize_subheader(self, offset, length):

~/.pyenv/versions/3.6.4/envs/ts/lib/python3.6/site-packages/pandas/io/sas/sas7bdat.py in _process_columntext_subheader(self, offset, length)
    432 
    433         if self.convert_header_text:
--> 434             cname = cname.decode(self.encoding or self.default_encoding)#cname.decode(self.encoding or self.default_encoding,'ignore')
    435         self.column_names_strings.append(cname)
    436 

UnicodeDecodeError: 'gbk' codec can't decode byte 0x8c in position 0: illegal multibyte sequence
```
[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]

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#### Expected Output

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None

pandas: 0.22.0
pytest: None
pip: 9.0.2
setuptools: 28.8.0
Cython: None
numpy: 1.14.0
scipy: None
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>




the  code fired issue,
```
        if self.convert_header_text:
            cname = cname.decode(self.encoding or self.default_encoding)
```
if change to

```
        if self.convert_header_text:
            cname = cname.decode(self.encoding or self.default_encoding,'ignore')
```
got 
![image](https://user-images.githubusercontent.com/3222056/37758430-2b022410-2deb-11e8-931e-fc818aa51898.png)


if change to 
```
       # if self.convert_header_text:
       #     cname = cname.decode(self.encoding or self.default_encoding)
```
got 
![image](https://user-images.githubusercontent.com/3222056/37758848-a7428ea6-2dec-11e8-80ff-7b77760aa3a7.png)

and decode columns with the following code,
```
col=df1.columns.tolist()
col = [x.decode('GBK', 'ignore') for x in col]
df1.columns=pd.Index(col)
```
got the correct one,
![image](https://user-images.githubusercontent.com/3222056/37758868-be2d973c-2dec-11e8-8b4c-36269dee5a38.png)


btw, 
[sas7bdat](https://pypi.python.org/pypi/sas7bdat) works well.
![image](https://user-images.githubusercontent.com/3222056/37759396-7404db82-2dee-11e8-8551-6a6a016871d7.png)
