Metadata should detect null boolean field type.

rest_framework/metadata.py

@@ -36,6 +36,7 @@ class SimpleMetadata(BaseMetadata):
     label_lookup = ClassLookupDict({
         serializers.Field: 'field',
         serializers.BooleanField: 'boolean',
+        serializers.NullBooleanField: 'boolean',
         serializers.CharField: 'string',
         serializers.URLField: 'url',
         serializers.EmailField: 'email',

tests/test_metadata.py

 from __future__ import unicode_literals
-from rest_framework import exceptions, serializers, status, views, versioning
+from rest_framework import exceptions, metadata, serializers, status, views, versioning
 from rest_framework.request import Request
 from rest_framework.renderers import BrowsableAPIRenderer
 from rest_framework.test import APIRequestFactory
@@ -207,3 +207,8 @@ class TestMetadata:
         scheme = versioning.QueryParameterVersioning
         view = ExampleView.as_view(versioning_class=scheme)
         view(request=request)
+
+    def test_null_boolean_field_info_type(self):
+        options = metadata.SimpleMetadata()
+        field_info = options.get_field_info(serializers.NullBooleanField())
+        assert field_info['type'] == 'boolean'