Fix a compiler error

So I realised where that attr parameter was used but they were actually default constructed strings so they really did have no use. I removed them to prevent a compilation error (oops :( ). 

One thing I noticed is that there is a lot of duplication of code for integral types using a macro. Would fixing that be desirable? Because if so then I wouldn't mind fixing that using the current `JSONXX_COMPILER_HAS_CXX11` macro and some slight template metaprogramming.