Make header() method case-insensitive

[ybr-nx]

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

HTTP 1/1 - RFC7230

Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared in a case-insensitive fashion.

HTTP2 - RFC7540