[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)>'
```

spec/requests/api/jobs_spec.rb

@@ -281,7 +281,7 @@ describe API::Jobs do
           get_artifact_file(artifact)
 
           expect(response).to have_gitlab_http_status(200)
-          expect(response.headers)
+          expect(response.headers.to_h)
             .to include('Content-Type' => 'application/json',
                         'Gitlab-Workhorse-Send-Data' => /artifacts-entry/)
         end
@@ -311,7 +311,7 @@ describe API::Jobs do
 
       it 'returns specific job artifacts' do
         expect(response).to have_gitlab_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(job.artifacts_file.file.file)
       end
     end
@@ -462,7 +462,7 @@ describe API::Jobs do
           end
 
           it { expect(response).to have_http_status(:ok) }
-          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

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

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