Remove Empty Request Headers

[mbande]

it seems that django's request.META, adds many http headers with empty values if they does not exist in client request. e.g. client send a request without Content-Type header, but request.META contains a CONTENT_TYPE key with empty string value.
i think revproxy should not send this headers to upstreams, as it does not make sense to send Content-Type: '' to a web server

[brianmay]

Pull requests welcome :-)

[seocam]

This is something we want for 1.0 (long coming milestone).
I'd say it's a duplicate of #33.

[petrprikryl]

Just trying PgHero behind proxy and got error: "" is not a valid MIME type in mime_type.rb.

I have tried request_payload = None based on #99 but it didn't help. So I wouldn't consider this issue as duplicate od #33.

PgHero is working just fine with patch from #94.