GenericValue::operator= : fixup assignment operator

While MSVC doesn't like the explicit `.template operator=<...>` syntax
(see 4f40ed6), Clang 3.5 complains about the absence of it:

In file included from ../../test/perftest/rapidjsontest.cpp:6:
../../include/rapidjson/document.h:504:18: error: use 'template' keyword to treat 'operator =' as a dependent template name
                return (*this).operator=<StringRefType>(str);
                               ^
                               template

Delegate both operator=(StringRefType) and operator=(T) to operator(GenericValue&).
Merge pull request #63 from pah/fixes/value-assign

GenericValue::operator= : fixup assignment operator (again)