fixed Content-Type and Content-Length for FCGI and NGINX setup

\Input::server('Content-Type') should return the value of Content-Type field if available.
However, \Input::server('Content-Type') always returns null with lighttpd and mod_cgi.

In \Input::headers(), \Input::server() is called when getallheaders() is missing.
\Input::server() should be called with an underscored name, but actually with an hyphened name.
For example, \Input::server('Content-Type') is wrong and must be \Input::server('CONTENT_TYPE').
That is as well about Content-Length.

This bug should reproduce for FCGI and NGINX setup or other setups that do not have getallheaders().

Signed-off-by: Udagawa Kaito <umireon@gmail.com>