Fixed #17713 -- Renamed BaseDatabaseFeatures.allows_primary_key_0 to allows_auto_pk_0.

MySQL does allow primary key with value 0. It only forbids autoincrement
primary key with value 0.

Thanks Claude Paroz for the report.

django/db/backends/__init__.py

@@ -613,8 +613,8 @@ class BaseDatabaseFeatures(object):
     # Is there a 1000 item limit on query parameters?
     supports_1000_query_parameters = True
 
-    # Can an object have a primary key of 0? MySQL says No.
-    allows_primary_key_0 = True
+    # Can an object have an autoincrement primary key of 0? MySQL says No.
+    allows_auto_pk_0 = True
 
     # Do we need to NULL a ForeignKey out, or can the constraint check be
     # deferred

django/db/backends/mysql/base.py

@@ -177,7 +177,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_date_lookup_using_string = False
     supports_timezones = False
     requires_explicit_null_ordering_when_grouping = True
-    allows_primary_key_0 = False
+    allows_auto_pk_0 = False
     uses_savepoints = True
     atomic_transactions = False
     supports_check_constraints = False