Remove to_ary from Response

[tenderlove]

Responses are not arrays, so we should not allow them to be implicitly
coerced to an array. If you would like you response to be converted to
an array, you must explicitly do it with the to_a method.

This also prevents creation of unnecessary body proxies

refs #419

[tenderlove]

I was able to confirm that without removing this line the flatten tests I added would go in to an infinite loop.

[dblock]

Looks like this will break Grape, ruby-grape/grape#1955. I haven't dug deep here, @dnesteryuk can probably provide an explanation.

[delner]

It does indeed break Grape; middleware/base.rb doesn't coerce the response it gets to an Array which eventually causes Router#cascade? to receive an Array with [#<Rack::Response>, nil nil].

This issue explains the effect on Grape: ruby-grape/grape#1966