Use unicode_literals in some tests missing it

These tests sent some Python 2 `str`s to unidecode via taggit, which
raised a RuntimeWarning. These strings should be unicode, and are
unicode when they come from Django outside of the tests.

unicode_literals should be added to all Python files to ensure
consistent handling of strings across Python versions, but that is a
larger and more controversial change.

wagtail/api/v2/tests/test_documents.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import json
 
 import mock

wagtail/api/v2/tests/test_images.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import json
 
 import mock

wagtail/contrib/wagtailapi/tests/test_documents.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import json
 
 import mock

wagtail/contrib/wagtailapi/tests/test_images.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import json
 
 import mock

wagtail/wagtailcore/tests/test_page_model.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import datetime
 import json