Doesn't enforce UTF-8 encoding in Response object if the string is frozen.

Elasticsearch responses are UTF-8 encoded.
Cloning the string instead could be very expensive.

Fixes #63

Author: picandocodigo

elasticsearch-transport/lib/elasticsearch/transport/transport/response.rb

@@ -28,7 +28,7 @@ class Response
         # @param headers [Hash]    Response headers
         def initialize(status, body, headers={})
           @status, @body, @headers = status, body, headers
-          @body = body.force_encoding('UTF-8') if body.respond_to?(:force_encoding)
+          @body = body.force_encoding('UTF-8') if body.respond_to?(:force_encoding) && !body.frozen?
         end
       end