Attempting to write to HEAD response hangs the request

[cesarblum]

Run the sample app without any connection filters and send a HEAD request and leave the connection open (this part is important). The client will hang waiting for a response.

This issue was introduced in e7e6b89 when we started ignoring writes to HEAD responses. The problem is that writing a response will make Frame.HasResponseStarted true, skipping the call to ProduceEndAwaited from ProduceEnd and therefore skipping the flush call in that method.

I caught this while working on #1139. When I changed all tests to call Send instead of SendEnd, ResponseTests.HeadResponseCanContainContentLengthHeaderButBodyNotWritten started timing out on TestConnection.Receive.

[muratg]

Approved by @Eilon, so triaging this one in.