Use consistent case for HTTP header fields, according to [RFC 7231](https://tools.ietf.org/html/rfc7231).

Thank you for your contribution and your issue, it's really appreciated. I had a doubt about this point and I wanted to verify about the HTTP headers.

But in the previous version of this RFC7230: https://www.w3.org/Protocols/rfc2616/rfc2616.html

Here is the section 4.2 "Message Headers" of this previous RFC: https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2
where you have "Field names are case-insensitive".

We can see the RFC2616 was replaced by multiple RFCs (7230-7237) and in the RFC 7230, there is a section 3.2 where there is the description of the header fields.

In the section 3.2 of RFC7230:

3.2. Header Fields
"
Each header field consists of a case-insensitive field name followed
by a colon (":"), optional leading whitespace, the field value, and
optional trailing whitespace.
"

https://tools.ietf.org/html/rfc7230#section-3.2

So in this case, it's case-insensitive.

I think we can close this issue and the associated Pull Request.

Have a nice day and thank you,

As stated on the PR, changing the code isn't going to be worth it, but updating the docs would be nice.

Please see the changes and my reply in the PR.

Closing as "not a bug" as discussed in the PR.

Anyway thanks Géry for the work on this, even if not merged it's appreciated!