Updated Rack::Throttle::Limiter#http_error to return HTTP body as an array

On Ruby 1.9.3 and Rails 3.2.x, the server was returning empty response, due to:

    !! Unexpected error while processing request: undefined method `each' for "403 Forbidden (Rate Limit Exceeded)\n":String

See <http://rack.rubyforge.org/doc/files/SPEC.html>, "The Body itself should not be an instance of String, as this will break in Ruby 1.9."