Add test that schema_name with underscores can be created. Refs djang…

…o-tenants#829

according to the description in django-tenants#829 Postgres will double quote
schema names which contain underscores.

django_tenants/tests/test_tenants.py

@@ -310,7 +310,9 @@ def test_tenant_schema_creation_with_only_numbers_name(self):
 
     def test_tenant_schema_creation_with_special_chars(self):
         """Tests using special characters in schema name."""
-        schema_names = ('test-hyphen', 'test@at', 'test`backtick')
+        schema_names = ('test-hyphen', 'test@at', 'test`backtick',
+            'will_be_double_quoted_by_postgres_bc_of_underscores',
+        )
 
         Client = get_tenant_model()
         for schema_name in schema_names: