This commit fixes an issue with Rack where it erroneously assumes
that a POST request with no Content-Type should be treated as
"application/x-www-form-urlencoded" or "multipart/form-data" causing
failures when binary data is posted.
This is related to the following Sinatra issue:
Section 7.2.1 of the HTTP/1.1 spec
Any HTTP/1.1 message containing an entity-body SHOULD include a
Content-Type header field defining the media type of that body.
If and only if the media type is not given by a Content-Type field,
the recipient MAY attempt to guess the media type via inspection of
its content and/or the name extension(s) of the URI used to identify
the resource. If the media type remains unknown, the recipient SHOULD
treat it as type "application/octet-stream".