Make NullBooleanField subclass BooleanField

This removes a lot of the redundancy that was in place becuase we
were not doing this. This maintains the `None` initial value that
was previously present, as well as disallowing `allow_null` to be
passed in.

rest_framework/fields.py

@@ -740,56 +740,14 @@ class BooleanField(Field):
         return bool(value)
 
 
-class NullBooleanField(Field):
+class NullBooleanField(BooleanField):
-    default_error_messages = {
-        'invalid': _('Must be a valid boolean.')
-    }
     initial = None
-    TRUE_VALUES = {
-        't', 'T',
-        'y', 'Y', 'yes', 'YES',
-        'true', 'True', 'TRUE',
-        'on', 'On', 'ON',
-        '1', 1,
-        True
-    }
-    FALSE_VALUES = {
-        'f', 'F',
-        'n', 'N', 'no', 'NO',
-        'false', 'False', 'FALSE',
-        'off', 'Off', 'OFF',
-        '0', 0, 0.0,
-        False
-    }
-    NULL_VALUES = {'null', 'Null', 'NULL', '', None}
 
     def __init__(self, **kwargs):
         assert 'allow_null' not in kwargs, '`allow_null` is not a valid option.'
         kwargs['allow_null'] = True
         super().__init__(**kwargs)
 
-    def to_internal_value(self, data):
-        try:
-            if data in self.TRUE_VALUES:
-                return True
-            elif data in self.FALSE_VALUES:
-                return False
-            elif data in self.NULL_VALUES:
-                return None
-        except TypeError:  # Input is an unhashable type
-            pass
-        self.fail('invalid', input=data)
-
-    def to_representation(self, value):
-        if value in self.NULL_VALUES:
-            return None
-        if value in self.TRUE_VALUES:
-            return True
-        elif value in self.FALSE_VALUES:
-            return False
-        return bool(value)
-
-
 # String types...
 
 class CharField(Field):