Wrap json.Encoder returned by RequestEncoder to assign GetBody on the http.Request

[duckbrain]

Closes #3725

Prior to this change, the following error could occur with the generated client code when connecting over http2

http2: Transport: cannot retry err [http2: Transport received Server's graceful shutdown GOAWAY] after Request.Body was written; define Request.GetBody to avoid this error 

This PR does as the message suggests and defines Request.GetBody

[raphael]

This looks great, thank you. The only thing might be to add comments to explain the code, something like:

1. In the RequestEncoder function (around lines 181-184):

Before:

 enc := new(jsonEncoder) r.Body = enc r.GetBody = enc.GetBody return enc

After:

 enc := new(jsonEncoder) r.Body = enc // GetBody enables request retry on HTTP/2 connections when the server // sends GOAWAY during graceful shutdown. Without GetBody, the HTTP transport // cannot retry because the request body has already been consumed. r.GetBody = enc.GetBody return enc

2. For the jsonEncoder struct (around lines 185-188):

 Before: type jsonEncoder struct { b []byte r bytes.Reader }

After:

 // jsonEncoder implements io.ReadCloser and provides GetBody functionality // to support HTTP/2 request retries during server graceful shutdown (GOAWAY). type jsonEncoder struct { b []byte // encoded JSON bytes r bytes.Reader // reader for the encoded bytes }

3. For the GetBody method (around lines 211-215):

Before:

 func (je *jsonEncoder) GetBody() (io.ReadCloser, error) { if len(je.b) == 0 { return nil, errEncodeNotCalled	} return io.NopCloser(bytes.NewReader(je.b)), nil }

After:

 // GetBody returns a new reader of the encoded bytes, enabling request retries. // This is required for HTTP/2 connections to handle server GOAWAY during graceful shutdown. func (je *jsonEncoder) GetBody() (io.ReadCloser, error) { if len(je.b) == 0 { return nil, errEncodeNotCalled	} return io.NopCloser(bytes.NewReader(je.b)), nil }

[duckbrain]

Sorry, for the delay. I've now got the comments added and satisfied the CI checks.

[raphael]

Looks great, thank you!