[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/v3/builds_spec.rb b/spec/requests/api/v3/builds_spec.rb
index 00f067889a0..485d7c2cc43 100644
--- a/spec/requests/api/v3/builds_spec.rb
+++ b/spec/requests/api/v3/builds_spec.rb
@@ -232,7 +232,7 @@ describe API::V3::Builds do
 
           it 'returns specific job 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)
             expect(response.body).to match_file(build.artifacts_file.file.file)
           end
         end
@@ -332,7 +332,7 @@ describe API::V3::Builds do
           end
 
           it { expect(response).to have_http_status(200) }
-          it { expect(response.headers).to include(download_headers) }
+          it { expect(response.headers.to_h).to include(download_headers) }
         end
 
         context 'when artifacts are stored remotely' do