* Fixed HTTP/2 support for the proxy middleware

http.Transport instances whose TLSClientConfig, Dial, or DialTLS field
is non-nil will be configured without HTTP/2 support by default.

This commit adds the proper calls to http2.ConfigureTransport()
everywhere a http.Transport is created and thus fixes HTTP/2 in the
proxy middleware whenever insecure_skip_verify or keepalive is provided.

* Added HTTP/2 support check to TestReverseProxyInsecureSkipVerify