Accessing the headers for outgoing requests.

Since the headers to be attached to each outgoing request are to be
saved, provide a way to view what has been set for them.
Adding headers testing.

Some quick unit tests asserting default behaviours, etc.
Comments fixes.
Moving default headers.
Removing the extra comment.
String formatting.

Missed in the last batch of amending formatting to the unindexed syntax.
Abstracting the GET request.
Version updates.
Creating a Gist from the URL works with HTTP.

Originally this failed with HTTP addresses instead of the "real" HTTPS
versions. It now accepts HTTP addresses that match.
Technically not a number.

It's just an ID, not a number.