Add missing proxy wrapper method

This should fix compiler ambiguities when wrapping proxy objects.

Author: kevinushey

inst/include/Rcpp/macros/interface.h

@@ -30,6 +30,10 @@ __CLASS__( const __CLASS__& other ){                                           \
 __CLASS__& operator=(const __CLASS__& rhs) {                                   \
     return Storage::copy__(rhs) ;                                              \
 }                                                                              \
+template <typename Proxy>                                                      \
+__CLASS__( const GenericProxy<Proxy>& proxy ){                                 \
+    Storage::set__( proxy.get() ) ;                                            \
+}
 
 #define RCPP_API_CLASS(__CLASS__)                                              \
 template < template <class> class StoragePolicy > class __CLASS__ :            \