Avoid HTTP Header collisions.

Even though the HTTP header keys are defined in the spec they might be
expanded/changed in the future.

The proposal is to follow the SPDY/HTTP2 format and
prefix the request/response line headers with a colon.

The colon headers for requests are:

* `:method`
* `:path`

The colon header for responses is:

* `:status`

See: https://aprescott.com/posts/spdy-colon-headers