Fix default Content-Type (RFC2616 7.2.1)

This patch fixes the behaviour of `Content-Type` header if it was not included in the request according to [RFC 2616 section 7.2.1](http://www.w3.org/Protocols/rfc2616/rfc2616-sec7.html#sec7.2.1). It also fixes sinatra/sinatra#804 issue.