[XMLHttpRequest] Lowercasing request headers

[barakcoh]

Seems #1381 is lowercasing all HTTP request headers in order to prevent duplicates. While the HTTP spec indeed specifies HTTP headers should be case insensitive, in the real world some server side implementations do not honor the case insensitivity.

Unfortunately A single mandatory case-sensitive header (e.g. Authorization) whose implementation is outside the reach of the RN developer can render RN completely useless.

Is it possible to deduplicate headers without changing the header names?

cc @philikon

[barakcoh]

Closing, as it seems both the fetch polyfill and superagent are both also lowercasing request headers, so it's not really an AN issue.