Fix lowercase "content-type" was ignored

[nangchan]

When specifying the header Content-Type, only uppercase "Content-Type" works but lowercase "content-type" is ignored and overridden by g:vrc_header_content_type.

Solution:
When the user specifies "content-type" the plugin will automatically uppercase to "Content-Type" to match the existing logic.

[riccione]

Check Michael Potter pull request 3179c79

Header names are not case sensitive.

From RFC 2616 - "Hypertext Transfer Protocol -- HTTP/1.1", Section 4.2, "Message Headers":

Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.

The updating RFC 7230 does not list any changes from RFC 2616 at this part.
https://stackoverflow.com/a/5259004