change the inhibition value of *_SAVE_AS to ''

Previously, the documentation claimed the value of None for this purpose
even though False was used for certain defaults. The values False and
None cause warnings to be emitted from URLWrapper._from_settings though,
so the new way of inhibiting page generation is to set a *_SAVE_AS value
to the empty string.
Merge pull request #1298 from th3aftermath/feature-inhibit-save_as

Change the inhibition value of *_SAVE_AS to ''