fix FoldersController#api_index pagination

@context is always nil. found by rails 3 specs

Change-Id: I37acb0edfc80987fa414f11bc338b07bebb79364
Reviewed-on: https://gerrit.instructure.com/30723
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>