Resolved ambiguity due to implicit tuple constructor in libc++.

See:
https://gist.github.com/derrickturk/ca48c1b93c4a8e28b5f4
http://open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3680.html
http://llvm.org/bugs/show_bug.cgi?id=15299
http://stackoverflow.com/questions/24253782/template-default-argument-sfinae-ambiguous-to-clang-fine-for-g