Incorrect status code returned when being rate limited

Issue Summary

It appears the Confluence rate limiting of the CQL search endpoint, `/rest/api/search?cql=`, is flawed with respect to the returned status code since 400 responses are observed when the endpoint is pushed hard.

Steps to Reproduce

1. Create an app that invokes the CQL search endpoint repeatedly in quick succession until a non 200 response is returned. 
2. Observe the non 200 response code.

Expected Results

The response status code at step 2 is 429.

Actual Results

The response status code at step 2 is 400.

Workaround

None