Fix set_cdata_mode() for Python 2.7.5, where set_cdata_mode() API changed.

This is not part of the public API, so they're allowed to do that.
(In fact, the API changed in order to fix the very same bug that
required me to override set_cdata_mode() here, so one day this whole
override will be able to go away.)

Test plan:
Fingers crossed