Added '/api/v3/projects/:id/merge_requests/:merge_request_id/closes_issues' route in the API

Added some documentation for it

Added to changelog

Added curl example and an attribute table

Moved the api route definition from "lib/api/issues.rb" to "lib/api/merge_requests.rb"

Fixed the attributes and changed the documentation to be at "merge_requests.md" too

Changed generic titles to more specific titles

added an underscore

added tests. it depends on a newer version of gitlab-test project
I'm doing a  since I need to add a branch to the `gitlab-test` repo

removed the before

using 'iid' instead of 'id' in the description to reference the issues. that makes the tests pass

Removed the 'closes-issues' key from test_env. so it should pass the tests

Moved the two initializations to the describe block

Changed the changelog