For the `request()` and `get()` functions. I could never
really understand why these two functions go through agent
first... Especially since the user could be passing `agent: false`
or a different Agent instance completely, in which `globalAgent`
will be completely bypassed.

Moved the relevant logic from `Agent#request()` into the
`ClientRequest` constructor.

Incidentally, this commit fixes #7012 (which was the original
intent of this commit).

Expose the list of supported HTTP methods as a property on the 'http'
module object.

Fixes #6422.

Noticed by @bnoordhuis in https://github.com/joyent/node/pull/5991#discussion_r5575946

Commit 38149bb changes http.get() and http.request() to escape unsafe
characters. However, that creates an incompatibility with v0.10 that
is difficult to work around: if you escape the path manually, then in
v0.11 it gets escaped twice. Change lib/http.js so it no longer tries
to fix up bad request paths, simply reject them with an exception.

The actual check is rather basic right now. The full check for illegal
characters is difficult to implement efficiently because it requires a
few characters of lookahead. That's why it currently only checks for
spaces because those are guaranteed to create an invalid request.

Fixes #5474.