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.
Add other tests to boost the coverage of Conn.Plug
Merge pull request #129 from whatyouhide/plug-conn-coverage

Plug conn coverage