Update Symbol From_Ruby#is_convertible to note converting from string…

… is a typecast.

rice/cpp_api/Symbol.ipp

@@ -78,7 +78,7 @@ namespace Rice::detail
           return Convertible::Exact;
           break;
       case RUBY_T_STRING:
-          return Convertible::Exact;
+          return Convertible::TypeCast;
           break;
         default:
           return Convertible::None;