Header names should not be case sensitive

[deki]

ServerlessHttpServletRequest currently stores and retrieves headers in a case sensitive way. Header names should not not be treated case sensitive though.
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.

Examples: request.getHeader("User-Agent") works, request.getHeader("user-agent") returns no result and vice versa.

Tomcat converts all headers to lower case directly but in my view making lookup case insensitive would already fix it.