UnPackTo disable merge by default (#7527)

* UnPackTo disable merge by default

* avoid double free in test

* remove merge parameter

* remove shrink to fit

tests/test.cpp

@@ -1409,6 +1409,7 @@ int FlatBufferTests(const std::string &tests_data_path) {
   MutateFlatBuffersTest(flatbuf.data(), flatbuf.size());
 
   ObjectFlatBuffersTest(flatbuf.data());
+  UnPackTo(flatbuf.data());
 
   MiniReflectFlatBuffersTest(flatbuf.data());
   MiniReflectFixedLengthArrayTest();