Dir fails on dataframes with pathological column names
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'\ud83d': []})
_ = dir(df)
```

```python-traceback
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-3-4e949fe17c82> in <module>
----> 1 _ = dir(df)

~/miniconda/envs/dev/lib/python3.7/site-packages/pandas/core/accessor.py in __dir__(self)
     37         """
     38         rv = set(dir(type(self)))
---> 39         rv = (rv - self._dir_deletions()) | self._dir_additions()
     40         return sorted(rv)
     41

~/miniconda/envs/dev/lib/python3.7/site-packages/pandas/core/generic.py in _dir_additions(self)
   5110         If info_axis is a MultiIndex, it's first level values are used.
   5111         """
-> 5112         additions = {c for c in self._info_axis.unique(level=0)[:100]
   5113                      if isinstance(c, string_types) and isidentifier(c)}
   5114         return super(NDFrame, self)._dir_additions().union(additions)

~/miniconda/envs/dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in unique(self, level)
   1999         if level is not None:
   2000             self._validate_index_level(level)
-> 2001         result = super(Index, self).unique()
   2002         return self._shallow_copy(result)
   2003

~/miniconda/envs/dev/lib/python3.7/site-packages/pandas/core/base.py in unique(self)
   1312         else:
   1313             from pandas.core.algorithms import unique1d
-> 1314             result = unique1d(values)
   1315
   1316         return result

~/miniconda/envs/dev/lib/python3.7/site-packages/pandas/core/algorithms.py in unique(values)
    360
    361     table = htable(len(values))
--> 362     uniques = table.unique(values)
    363     uniques = _reconstruct_data(uniques, dtype, original)
    364     return uniques

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.StringHashTable.unique()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.StringHashTable._unique()

UnicodeEncodeError: 'utf-8' codec can't encode character '\ud83d' in position 0: surrogates not allowed
```

#### Problem description

Dir fails on dataframes with pathalogical column names

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Darwin
OS-release: 18.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.1
pytest: 3.10.1
pip: 18.1
setuptools: 40.6.2
Cython: None
numpy: 1.15.4
scipy: 1.1.0
pyarrow: 0.11.1
xarray: 0.11.3
IPython: 7.2.0
sphinx: 1.8.4
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: 0.2.0
fastparquet: 0.1.6
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
