[tuple.general] Fix return type of forward_as_tuple

In [tuple.general], change the return type of forward_as_tuple
from tuple<Types...>, 'Types' in italic monoface, to
tuple<Types&&...> in normal monoface, as it appears in
[tuple.creation].
Merge pull request #36 from smithpaul/forward_as_tuple-fix

[tuple.general] Fix return type of forward_as_tuple. Thanks to Paul Smith!