Fix some usage of rvalue references

[0x1997]

std::move is missing in a few places.

[coveralls]

Coverage decreased (-0.001%) to 99.936% when pulling be54944 on 0x1997:missing-move into f28203c on miloyip:master.

[pah]

These explicit std::moves are not needed due to RapidJSON's default use of move semantics. They are even harmful, because they lead to an infinite recursion. ;-)

[0x1997]

@pah I see. Thanks.