collect the body and make assert against the collected body

Add `ActionController::Metal#set_request!` to set a request
on controller instance without calling dispatch.

Rails includes a single character body to a head(:no_content) response to work around an old Safari bug where headers were ignored if no body sent.

This patch brings the behavior slightly closer to spec if :no_content/204 is explicity requested via a head only response.

Status comparison done on symbolic and numeric values

Not returning any content when responding with head and limited to a status code that explicitly states no content will be returned - 100..199, 204, 205, 304.

A test case to ensure that AC::Metal#response_body= always wraps the given value in an Array in both Ruby 1.8 and 1.9 (refs #3581)

[#5393 state:committed]

[#4645 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>