Attempt to load dictionary for class

This is a ROOT6 specific bug fix that allows two more framework unit tests to pass.
If an attempt is made to construct a TypeWithDict from a class name, and the dictionary for the class has not yet been loaded, a default TypeWithDict will be constructed, rather than one for the class in question.
The fix calls TClass::GetClass for the class name, which will load the dictionary if RootAutoLibraryLoader is in use.
Then, the proper TypeWithDict can be constructed