Required fields need to be verified before write

It seems as though the 'required' keyword is not really enforced at least in C++ and possibly Python for v0.13.0. There are a few tickets there already. THRIFT-4946, THRIFT-5041, THRIFT-735, THRIFT-4241, THRIFT-455.

I don't mind providing a simple fix for C++ and an additional test case, but there should be an effort to look through whether or not this is broken across other languages / generator targets.