Excelwriter :  engine_kwargs error with xlsxwriter engine
I am tring to pass {'constant_memory' = True} with xlsxengine (to increase speed).

```
writer=pd.ExcelWriter('raw_data_dump.xlsx', engine='xlsxwriter', engine_kwargs={'constant_memory':True})
raw_data.to_excel(writer, sheet_name='Sheet1')
writer.save()
```

I get the following error :

```
Exception AttributeError: "'Workbook' object has no attribute 'fileclosed'" in <bound method Workbook.__del__ of <xlsxwriter.workbook.Workbook object at 0x10dcf68d0>> ignored
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-54-d2dacd68b48f> in <module>()
----> 1 writer=pd.ExcelWriter('raw_data_dump.xlsx', engine='xlsxwriter', engine_kwargs={'constant_memory':True})
      2 raw_data.to_excel(writer, sheet_name='Sheet1')
      3 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 'engine_kwargs'
```
