syscall/js does not allow []byte to be used in direct inputs to
its JavaScript manipulation methods since
https://github.com/golang/go/commit/bafe466a9537d8ea5ac5767504628803302ebb12.
Unfortunately, this use of a byte slice was missed, so any
uses of the WASM Roundtripper with a body will panic.
This ensures the byte slice is appropriately converted
before being passed to syscall.

Fixes #26349

Change-Id: I83847645d71ce310c1eee3decddbac990fae166b
GitHub-Last-Rev: 3914bda2ff457deba4b76182a48d26ce8948b4cb
GitHub-Pull-Request: golang/go#26350
Reviewed-on: https://go-review.googlesource.com/123537
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>