Handle POST data with no Content-Type properly

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: 
https://github.com/sinatra/sinatra/issues/514

Section 7.2.1 of the HTTP/1.1 spec 
(http://www.w3.org/Protocols/rfc2616/rfc2616-sec7.html#sec7.2.1) says:

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".