support google spreadsheet export *.ods

load an .ods file that export from google spreadsheet is broken.
TypeError: Argument must be bytes or unicode, got 'NoneType'
```
Traceback (most recent call last):
  File "./import_diff.py", line 405, in <module>
    doc = ezodf.opendoc(data_filename)
  File "/Library/Python/2.7/site-packages/ezodf/document.py", line 49, in opendoc
    return PackagedDocument(filemanager=fm, mimetype=mimetype)
  File "/Library/Python/2.7/site-packages/ezodf/document.py", line 191, in __init__
    self.filemanager.manifest.add('/', mimetype)
  File "/Library/Python/2.7/site-packages/ezodf/manifest.py", line 40, in add
    set_media_type_and_version(file_entry)
  File "/Library/Python/2.7/site-packages/ezodf/manifest.py", line 33, in set_media_type_and_version
    file_entry.set(CN('manifest:media-type'), media_type)
  File "lxml.etree.pyx", line 713, in lxml.etree._Element.set (src/lxml/lxml.etree.c:39488)
  File "apihelpers.pxi", line 520, in lxml.etree._setAttributeValue (src/lxml/lxml.etree.c:17682)
  File "apihelpers.pxi", line 1331, in lxml.etree._utf8 (src/lxml/lxml.etree.c:24677)
TypeError: Argument must be bytes or unicode, got 'NoneType'
```