Update docs on host matching in the `forward` macro
Merge pull request #127 from whatyouhide/forward-with-host-docs

Update docs on host matching in the `forward` macro
Add a bunch of tests to Plug.Conn

Code coverage tools suggested some untested sections of the `Plug.Conn` code.
This commit introduces tests for some of those sections, mainly the ones related
with raising a `Plug.Conn.AlreadySentException` with an already sent connection.
SVG Travis badge ftw

If you happen to look at stuff on a retina display, you'll thank me for this :).