Returns binary content (downloadable file), converted to a String
# File actionpack/lib/action_controller/test_process.rb, line 268
def binary_content
raise "Response body is not a Proc: #{body.inspect}" unless body.kind_of?(Proc)
require 'stringio'
sio = StringIO.new
body.call(self, sio)
sio.rewind
sio.read
end