Support case insensitive headers

[hfrances]

Dear Mr. Jonstodle: According to the official documentation, the HTTP headers are case-insensitive. I would be grateful if you could add this modification to your library.

Source: https://www.rfc-editor.org/rfc/rfc7230#section-3.2

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

Thanks in advance.