Export - Update Saved Mapping not setting contact_type column AND reset task portion of form when done

1. As of current trunk rev (5400) - when you try to update an existing Saved Export Mapping by adding another field, the field is added to civicrm_mapping_field BUT the contact_type column for the new row is NULL. Hence, when you reload the saved mapping you don't see the added field.

To recreate:

• Select some records (sub_type = Student) for export.
• Map Last Name, Student-GPA, Student-Household Income, check Save Mapping and Export.
• Go back to Search
• Now export again - but add another field (Student-SAT Score) and check Updated Saved mapping
• Query civicrm_mapping_field and check the last added record (contact_type will be NULL - should be Student)

2. Reset task part of form when Done with Export (check w/ Lobo on how to do this)
Currently, when you return to do a second export after saving a mapping, you get a "partial" load of the prior mapping. Also, if you then load a saved mapping, you get a "duplicate mapping" error because the "Save Mapping" checkbox is still checked.