[Rails5] Fix `undefined method 'downcase'` errors in requests specs

In Rails 5.0 headers are no longer inherited from Hash.
This commit fixes errors in requests specs like:

```
1) API::V3::Builds GET /projects/:id/builds/:build_id/artifacts job with artifacts when artifacts are stored locally authorized user returns specific job artifacts
    Failure/Error: expect(response.headers).to include(download_headers)

    NoMethodError:
      undefined method `downcase' for #<Hash:0x000055e8bccc23b0>
    # ./spec/requests/api/v3/builds_spec.rb:235:in `block (6 levels) in <top (required)>'
```

1 files changed, 2 insertions, 2 deletions

diff --git a/spec/requests/api/runner_spec.rb b/spec/requests/api/runner_spec.rb
index 17c7a511857..6c1c13c9d80 100644
--- a/spec/requests/api/runner_spec.rb
+++ b/spec/requests/api/runner_spec.rb
@@ -1330,7 +1330,7 @@ describe API::Runner do
 
               it 'download artifacts' do
                 expect(response).to have_http_status(200)
-                expect(response.headers).to include download_headers
+                expect(response.headers.to_h).to include download_headers
               end
             end
 
@@ -1345,7 +1345,7 @@ describe API::Runner do
 
                 it 'uses workhorse send-url' do
                   expect(response).to have_gitlab_http_status(200)
-                  expect(response.headers).to include(
+                  expect(response.headers.to_h).to include(
                     'Gitlab-Workhorse-Send-Data' => /send-url:/)
                 end
               end