Excelwriter :  engine_kwargs error with xlsxwriter engine
Issue related to #6831 

Passing directly the keyword argument, as suggested by @jtratner in #6831  also raises an error :

`writer=pd.ExcelWriter('raw_data_dump.xlsx', engine='xlsxwriter', constant_memory=True)
for asset_name, frame in export_fuel_data.groupby('asset_name'):
    frame.to_excel(writer, sheet_name=asset_name)
writer.save()`

Traceback:

```

Exception AttributeError: "'Workbook' object has no attribute 'fileclosed'" in <bound method Workbook.__del__ of <xlsxwriter.workbook.Workbook object at 0x108657110>> ignored

TypeError                                 Traceback (most recent call last)
<ipython-input-20-315e59384c18> in <module>()
----> 1 writer=pd.ExcelWriter('raw_data_dump.xlsx', engine='xlsxwriter', constant_memory=True)
      2 for asset_name, frame in export_fuel_data.groupby('asset_name'):
      3     frame.to_excel(writer, sheet_name=asset_name)
      4 writer.save()

/usr/local/Cellar/python/2.7.4/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/io/excel.pyc in __init__(self, path, engine, date_format, datetime_format, **engine_kwargs)
    724             **engine_kwargs)
    725 
--> 726         self.book = xlsxwriter.Workbook(path, **engine_kwargs)
    727 
    728     def save(self):

TypeError: __init__() got an unexpected keyword argument 'constant_memory' 
```
## INSTALLED VERSIONS

commit: None
python: 2.7.4.final.0
python-bits: 64
OS: Darwin
OS-release: 13.0.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: fr_BE.UTF-8

pandas: 0.13.1
Cython: 0.20.1
numpy: 1.8.1
scipy: 0.12.0
statsmodels: 0.5.0
IPython: 2.0.0
sphinx: 1.2.2
patsy: 0.2.1
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.2
bottleneck: None
tables: 3.1.0
numexpr: 2.3.1
matplotlib: 1.3.0
openpyxl: 1.6.2
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: 0.5.3
sqlalchemy: 0.8.2
lxml: 3.2.1
bs4: 4.1.3
html5lib: None
bq: None
apiclient: None
