fix(Request): Improve support for content-length under wsgiref

Under wsgiref, if no content-length header is provided, an empty
string is inserted. Update falcon.Request to handle this case,
as well as to more gracefully handle the case that the header is
set to any invalid value.
refactor(Request): Fix indentation of _parse_form_urlencoded

Remove the extra indentation level of the _parse_form_urlencoded
method's body.
Merge pull request #358 from kgriffs/wsgiref

fix(Request): Improve support for content-length under wsgiref