The default WASM RoundTripper is implemented using
the browser Fetch API. Some options don't readily map to
existing http.Request options, so we use the precedent
set by the TrailerPrefix constant to allow a user to configure
the "mode" and "credentials" options by supplying them
as headers in the http.Request.

Updates #26769

Change-Id: If42d24418c4ffb17211f57e36708cf460fb4c579
GitHub-Last-Rev: b230502084d628938cd50818d3d336f9f911d48d
GitHub-Pull-Request: golang/go#26784
Reviewed-on: https://go-review.googlesource.com/127718Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>