aws#1434

This is intended to get the following integration tests passing:

tests/integration/local/start_api/test_start_api.py
  TestServiceCorsSwaggerRequests
  TestServiceCorsGlobalRequests

Those tests use a
[CorsConfiguration](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-property-api-corsconfiguration.html)
and a [Cors
Global](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-specification-template-anatomy-globals.html),
respectively, to enable CORS.

When CORS is enabled, OPTIONS requests should send a 200 with the
specified CORS headers, even if no OPTIONS handler is explicitly stated
in the pre-transformation SAM template. I confirmed this by deploying
from a [SAM
template](https://github.com/douglasnaphas/play-sam/tree/master/cors-swagger)
similar to one of the failing integration tests (with Cors specified but
with no explicit OPTIONS handler), and observing that it does indeed
respond with a 200 and the CORS headers attached.

aws#1434

This adds an integration test to verify that templates with explicit
OPTIONS handlers actually have those handlers invoked on OPTIONS
requests. There has been an issue with OPTIONS requests returning 200
and not reaching an explicitly specified OPTIONS handler.
aws#1649 fixes that, so if the
integration test added in this commit passes (and fails on the develop
branch), it could be added to that PR.