[#1701] Move and clean up search index tests
[#1701] Add tests for date fields, including a failing one for #1701
[#1701] Don't index empty dates as the current one

When parsing an empty string, dateutil will always use a default one,
which by default is now(). We use here a fake date to see if that was
the case, and if so remove the value from the dict to index.
Merge branch 'master' into 1701-date-field-index

Conflicts:
	ckan/new_tests/lib/search/test_index.py
Merge branch '1701-date-field-index'