Fixes test-meta_box_models.php#148

- Test should return null if no data is passed in $_POST
Fixes test-meta_box_models.php#167

- Test required fields to be sent to the validator since #14
Fixes test-meta_box_models.php#187

- Date fields can validate to a taxonomy or a custom field, made inc/meta-box-models.php
  better at figuring out whether it is validating to custom fields or a taxonomy
Fixes test-meta_box_models.php#209

- Need to pass a field with the post_id now, validate returns a value, not an array
Fixes testInvalidNumericFieldExpectsDataRemoved

- Non-numeric data passed to a numeric field should return null, not void
- A field must now be passed to validate()
Re-group passing tests into @stable
Fixes TextFieldExpectsStringReturned

- validate now requires a field sent with the post ID
- validate now returns a sanitized value, not an array
Renamed test for text area fields given improper data

- If a textarea is given an array (or any field type for that matter)
  it should return null, not a WP_Error
Fixes a problem with special validators

- taxonomyselect, date, link, and other select fields should validate
  with special validation functions and then stop.
Fixes remaining erroring tests
Fixes validate_and_save test.