Change code to not assume ".xhtml" for application/xhtml+xml

mimetypes.guess_extension can sometimes return ".xht", see

>>> mimetypes.guess_all_extensions('application/xhtml+xml')
['.xhtml', '.xht']

cnx-epub shouldn't break in case extension is ".xht"
Merge pull request #16 from Connexions/fix-mimetypes-guess-extension

Change code to not assume ".xhtml" for application/xhtml+xml