mailing creation: illogical defaults, etc.

while diagnosing some oddities with how mailings were being handled at various steps, I started to realize that we're setting some values during the mailing construction that we really shouldn't be. for example – our set of default values when first creating the mailing record include setting the scheduled and approver ids and dates. that makes no sense – why would we set a scheduled date before the user even gets to that step in the wizard?

some of this is later corrected by some of the conditional logic that follows. but the whole CRM_Mailing_BAO_Mailing::create() method (which is called at every step in the wizard) needs to be reviewed and cleaned up.