Move last_updated(), date_created() and epoch() to methods.common.
Properly test that DATE_CREATED is in the document.
Default DATE_CREATED for single document GET.
Default DATE_CREATED/LAST_UPDATED on edit methods.

These fields need a default value if missing, so ETag will be computed
on the same representation of GET.
Test edit action on document with missing 'created' field.
Changelog update.
Merge branch 'issue123' into develop. Closes #123.