Throw InvalidOperationException if the user tries to modify response headers after the body starts

[Tratcher]

Sometimes users accidentally set response headers after the response starts and it can be very hard to debug why the client did not receive the header.
aspnet/Mvc#4065 (comment)

Throw an InvalidOperationException if the user tries to modify response headers after the first write to the body.

WebListener already does this. Helios used to as well.

[benaadams]

Readonly or full on disposed?

[Tratcher]

readonly, you may still want to enumerate and log them

[benaadams]

Is in #608