Introduce have_gitlab_http_status

So that whenever this failed:

    expect(response).to have_gitlab_http_status(200)

We see what's the response there. Here's an example:

```
1) API::Settings Settings PUT /application/settings custom repository storage type set in the config updates application settings
   Failure/Error: expect(response).to have_gitlab_http_status(200)
     expected the response to have status code 200 but it was 400. The response was: {"error":"password_authentication_enabled, signin_enabled are mutually exclusive"}
```

spec/requests/api/version_spec.rb

@@ -6,7 +6,7 @@ describe API::Version do
       it 'returns authentication error' do
         get api('/version')
 
-        expect(response).to have_http_status(401)
+        expect(response).to have_gitlab_http_status(401)
       end
     end
 
@@ -16,7 +16,7 @@ describe API::Version do
       it 'returns the version information' do
         get api('/version', user)
 
-        expect(response).to have_http_status(200)
+        expect(response).to have_gitlab_http_status(200)
         expect(json_response['version']).to eq(Gitlab::VERSION)
         expect(json_response['revision']).to eq(Gitlab::REVISION)
       end

spec/support/matchers/have_gitlab_http_status.rb

@@ -0,0 +1,14 @@
+RSpec::Matchers.define :have_gitlab_http_status do |expected|
+  match do |actual|
+    expect(actual).to have_http_status(expected)
+  end
+
+  description do
+    "respond with numeric status code #{expected}"
+  end
+
+  failure_message do |actual|
+    "expected the response to have status code #{expected.inspect}" \
+    " but it was #{actual.response_code}. The response was: #{actual.body}"
+  end
+end