Before this change, missing User-Agent and Accept headers were automatically
added on all platforms. Setting the User-Agent header forces the browser to
do a CORS preflight (see 1) which fails if the HTTP endpoint is not
configured appropriate. It's not neccesary to set either header as the
browser sets them and so this commit disables that functionality on the JS
target.
1: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#Simple_requests
(cherry picked from commit 8a4dccc4ce)
139185e543