ValidationError on query_string when running full_clean

[sycdan]

I was experimenting with a signal that runs full_clean before saving all model instances, and ran into this error for the easyaudit.models.RequestEvent model:

django.core.exceptions.ValidationError: {'query_string': ['This field cannot be blank.']}

I believe query_string should be flagged as blank=True: https://github.com/soynatan/django-easy-audit/blob/03e05bc94beb29fc3e4ff86e313a6fef4b766b4b/easyaudit/models.py#L76

I am hosting easyaudit in my repo because I had to work around another issue, so this was a quick fix in my case, but I do think this should be addressed in master.

Thanks! Great plugin, by the way. :)

[jheld]

Can you open a PR to adopt the change?