question: private boolean writeRequestHeadersEagerly ( ) { return callerWritesRequestBody && permitsRequestBody ( networkRequest ) && requestBodyOut == null ; }
target:  If the caller's control flow writes the request body, we need to create that stream immediately. And that means we need to immediately write the request headers, so we can start streaming the request body. (We may already have a request body if we're retrying a failed POST.)