[1.7.x] Fixed an inconsistency introduced in 547b1810.

mark_safe and mark_for_escaping should have been kept similar.

On Python 2 this change has no effect. On Python 3 it fixes the use case
shown in the regression test for mark_for_escaping, which used to raise
a TypeError. The regression test for mark_safe is just for completeness.

Backport of 5c5eb5fe from master.