net/http: Client support for Expect: 100-continue #3665
Description
The http Client should probably support Expect: 100-continue. The server does. It does introduce a new round-trip, though, so we should probably only send it when the data size exceeds some threshold. Probably the same threshold we use for consuming request bodies when a handler doesn't (maxPostHandlerReadBytes). I think we could see if we know the size of the request body (just as NewRequest does, checking for a few known types), but if the Body is non-nil and the ContentLength is <=0 (unknown), we instead slurp up the Body into a bytes.Buffer to see if it's >= someThreshold. If so, we replace the Body with a io.MultiReader(slurped, rest) and proceed to do a 100-continue. If not, we blast it away HTTP/1.0-style.