Add specs for pagination Link header

Add specs that check the 'Link' header for the inclusion of:

 - rel="first"
 - rel="last"
 - rel="prev"
 - rel="next"

Fixes gitlab-org/gitlab-ce#36618

Related to gitlab-com/infrastructure#2532

changelogs/unreleased/tc-git-tower-pagination-links.yml

+---
+title: Improve API pagination headers when no record found
+merge_request: 13629
+author: Jordan Patterson
+type: fixed

spec/lib/api/helpers/pagination_spec.rb

@@ -52,7 +52,13 @@ describe API::Helpers::Pagination do
           expect_header('X-Page', '1')
           expect_header('X-Next-Page', '2')
           expect_header('X-Prev-Page', '')
-          expect_header('Link', any_args)
+
+          expect_header('Link', anything) do |_key, val|
+            expect(val).to include('rel="first"')
+            expect(val).to include('rel="last"')
+            expect(val).to include('rel="next"')
+            expect(val).not_to include('rel="prev"')
+          end
 
           subject.paginate(resource)
         end
@@ -75,15 +81,52 @@ describe API::Helpers::Pagination do
           expect_header('X-Page', '2')
           expect_header('X-Next-Page', '')
           expect_header('X-Prev-Page', '1')
-          expect_header('Link', any_args)
+
+          expect_header('Link', anything) do |_key, val|
+            expect(val).to include('rel="first"')
+            expect(val).to include('rel="last"')
+            expect(val).to include('rel="prev"')
+            expect(val).not_to include('rel="next"')
+          end
+
+          subject.paginate(resource)
+        end
+      end
+    end
+
+    context 'when resource empty' do
+      describe 'first page' do
+        before do
+          allow(subject).to receive(:params)
+            .and_return({ page: 1, per_page: 2 })
+        end
+
+        it 'returns appropriate amount of resources' do
+          expect(subject.paginate(resource).count).to eq 0
+        end
+
+        it 'adds appropriate headers' do
+          expect_header('X-Total', '0')
+          expect_header('X-Total-Pages', '0')
+          expect_header('X-Per-Page', '2')
+          expect_header('X-Page', '1')
+          expect_header('X-Next-Page', '')
+          expect_header('X-Prev-Page', '')
+
+          expect_header('Link', anything) do |_key, val|
+            expect(val).to include('rel="first"')
+            expect(val).to include('rel="last"')
+            expect(val).not_to include('rel="prev"')
+            expect(val).not_to include('rel="next"')
+          end
 
           subject.paginate(resource)
         end
       end
     end
 
-    def expect_header(name, value)
-      expect(subject).to receive(:header).with(name, value)
+    def expect_header(*args, &block)
+      expect(subject).to receive(:header).with(*args, &block)
     end
 
     def expect_message(method)