test stubs require keys to be strings not symbols

For #928

README.md

@@ -923,6 +923,9 @@ Now, any time Geocoder looks up "New York, NY" its results array will contain on
         }
       ]
     )
+    
+Note: 
+  Keys must be strings not symbols when calling `add_stub` or `set_default_stub`. For example `'latitude' =>` not `:latitude =>`.
 
 
 Command Line Interface