bug: log spam with GOAWAY errors; txmit not retried on GOAWAY

[lizthegrey]

error sending libhoney event: Post \"https://api-kibble.honeycomb.io/1/batch/retriever-traces\": 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 is related to the work I did to fix content-length; GetBody is defined if you pass in a raw bytes.Buffer but not if you pass a subclass or embed of bytes.Buffer. #156 (comment)

[lizthegrey]

More specifically on this bug: while it prints a warning, we do have our external retry, but GOAWAY should be retried internally to http rather than needing us to externally retry. So it's better to support handling this directly by passing the bytes.Buffer directly into http.Request

[MikeGoldsmith]

Closed by #159