Allow insensitive header sending

[j-mendez]

Some older web servers require headers to be title case. We need to determine if a header starts with title case for only certain headers.
If it is insensitive the headers should not be lowercased by default?

[seanmonstar]

There is an option http1_title_case_headers. Is that enough for your needs?

[j-mendez]

Hi, @seanmonstar no we need to be able to have some headers lowercase and some caps. The google chrome browser sends headers this way and we need to mimic the real world handling of it since some websites have strict anti-bot defenses that require this.

[seanmonstar]

Ah, well in that case, this is blocked by support in hyper: hyperium/hyper#2695

That issue needs work to design a public API for the experimental feature, and then reqwest could support it.