Allow string + bytearray => bytearray. Fixes #780

src/runtime/str.cpp

@@ -342,11 +342,17 @@ extern "C" Box* strAdd(BoxedString* lhs, Box* _rhs) {
     }
 
     if (!PyString_Check(_rhs)) {
+        if (PyByteArray_Check(_rhs)) {
+            Box* rtn = PyByteArray_Concat(lhs, _rhs);
+            checkAndThrowCAPIException();
+            return rtn;
+        } else {
             // Note: this is deliberately not returning NotImplemented, even though
             // that would be more usual.  I assume this behavior of CPython's is
             // for backwards compatibility.
             raiseExcHelper(TypeError, "cannot concatenate 'str' and '%s' objects", getTypeName(_rhs));
         }
+    }
 
     BoxedString* rhs = static_cast<BoxedString*>(_rhs);
     return new (lhs->size() + rhs->size()) BoxedString(lhs->s(), rhs->s());