Fix bug with non-copyable map elements.

include/stream/streams/source.hpp

@@ -40,7 +40,7 @@ namespace stream { namespace detail {
   template <typename It, typename V = typename It::value_type>
   class range_stream {
   public:
-    typedef V & reference;
+    typedef typename It::reference reference;
 
     explicit range_stream(It b, It e) : begin_(b), end_(e) {}