Fix conn.assigns not available in view

The values assigned with `assign(conn, :key, value)` were not available in the view.

This PR let you use `assigns` as described in  #168 
@josevalim should those be stored in plug's private fields instead?

On Sat, Jul 19, 2014 at 12:11 PM, José Valim <notifications@github.com>
wrote:

>> @@ -157,6 +157,8 @@ defmodule Phoenix.Controller do
>>      layout       = Dict.get(assigns, :layout, "application")
>>      status       = Dict.get(assigns, :status, 200)
> Note to @chrismccord (not related to this PR). Mixing the assigns with other parameters like status and layout seems like a bad idea as the user may want to pass `status: :ready` to the view and it will be mixed by an actual response code.
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> https://github.com/phoenixframework/phoenix/pull/197/files#r15144639