make ResultItem#[] call 'to_sym' on supplied key to better support mu…

…lti_value fields.


Given a multi_field ``name`` with sub field ``ngrams`` and a query which highlights ``name.ngrams``, if we try to use the highlight with ``result.highlight['name.ngrams']`` we'll get nil even though inspect shows <Item name.ngrams: [".."]>. Doing ``result.highlight['name.ngrams'.to_sym] works as expected, but tire should do this for us.

lib/tire/results/item.rb

@@ -28,7 +28,7 @@ def method_missing(method_name, *arguments)
       end
 
       def [](key)
-        @attributes[key]
+        @attributes[key.to_sym]
       end
 
       def id