Reset Frame's IHttpRequestFeature.Headers and IHttpResponseFeature.Headers between requests

[khellang]

See #862 (comment)

Also, look into consolidating RequestHeaders and FrameRequestHeaders. See #862 (comment)

[cesarblum]

@muratg I think this is important enough for 1.0.0. @halter73 do you agree?

[muratg]

CC @mikeharder & @davidfowl for their thoughts as well.

[davidfowl]

Lets do it.

[halter73]

This change should be very simple even if it might not affect (m)any real apps.