fix(h1): ending close-delimited body should close

[hawkw]

PR #2264 introduced a bug where calling end_body on close-delimited
encoders would not actually close the connection. This is because the
call to encoder.end() now returns Ok(None) for close-delimited
encoders, to prevent the error from being propagated as though the user
body ended too early. However, when Ok is returned, the write state is
only advanced to Closed if the encoder returns true from is_last.
This means that when the body is close-delimited, ending the body does
nothing, leaving the connection in keepalive.

This causes a hang in Linkerd, where we proxy HTTP requests connections
by returning a client body in a response. If the upstream server closes
the connection, we need to rely on the close always being propagated to
the connection with our client. This bug resulted in the body closing
abruptly leaving the connection in a stuck state, instead.

This branch fixes this by special-casing close-delimited bodies so that
they will also set the write state to Closed when calling end_body,
even if the is_last bit has not been set.

Signed-off-by: Eliza Weisman eliza@buoyant.io