fix(server): error if Response code is 1xx

Returning a Response from a Service with a 1xx StatusCode is not
currently supported in hyper. It has always resulted in broken
semantics. This patch simply errors better.

- A Response with 1xx status is converted into a 500 response with no body.
- An error is returned from the `server::Connection` to alert about the
  bad response.

src/proto/mod.rs

@@ -148,7 +148,7 @@ pub trait Http1Transaction {
     type Outgoing: Default;
     fn parse(bytes: &mut BytesMut) -> ParseResult<Self::Incoming>;
     fn decoder(head: &MessageHead<Self::Incoming>, method: &mut Option<::Method>) -> ::Result<Option<h1::Decoder>>;
-    fn encode(head: MessageHead<Self::Outgoing>, has_body: bool, method: &mut Option<Method>, dst: &mut Vec<u8>) -> h1::Encoder;
+    fn encode(head: MessageHead<Self::Outgoing>, has_body: bool, method: &mut Option<Method>, dst: &mut Vec<u8>) -> ::Result<h1::Encoder>;
 
     fn should_error_on_parse_eof() -> bool;
     fn should_read_first() -> bool;