Fixed malloc cast warning in gh_stringWithFormat:arguments:

This code was allocating a dynamic C array of id and casting it to a char pointer. The static analyzer said:

> Result of 'malloc' is converted to a pointer of type 'char', which is incompatible with sizeof operand type 'NSString *'

I was a little confused, too, so I changed around some of the types and now the compiler, static analyzer, and I are happy.