Fix UniversalHandlers resolving the response before headers are complete

[timostamm]

#582 and #575 and surfaced an issue with the UniversalHandler implementations for the Connect, gRPC, and gRCP-web protocols: The function returns a promise for a UniversalServerResponse that resolves immediately, before the RPC implementation is invoked.

This causes some server implementations (fetch API handlers, and createRouterTransport, but not @bufbuild/connect-node) to write the response headers before the RPC implementation had a chance to add its own response headers.

The fix is to wait until the first response body byte is produced before resolving the response.

[dimitropoulos]

#590 passing CI confirms that this works! None of my feedback is super blocking.

[dimitropoulos]

update: inspired by https://github.com/bufbuild/connect-es/pull/588/files#r1171944922 I went ahead and checked the header values the same way for the interceptor tests in 0526f9a and it doesn't pass, but I'm not sure why? Could there be a sneaky bug somewhere still, or did I just do something wrong?

[timostamm]

I went ahead and checked the header values the same way for the interceptor tests in 0526f9a and it doesn't pass, but I'm not sure why?

Looks like you set "stream-response-header", but expect "unary-response-header".

[dimitropoulos]

Looks like you set "stream-response-header", but expect "unary-response-header".

oh phew! thanks!!