Fix can't modify frozen String errors

Use unary plus as recommended by Performance/UnfreezeString cop

spec/features/ics/dashboard_issues_spec.rb

@@ -93,7 +93,7 @@ describe 'Dashboard Issues Calendar Feed' do
 
         expect(body).to have_text("SUMMARY:test title (in #{project.full_path})")
         # line length for ics is 75 chars
-        expected_description = "DESCRIPTION:Find out more at #{issue_url(issue)}".insert(75, ' ')
+        expected_description = (+"DESCRIPTION:Find out more at #{issue_url(issue)}").insert(75, ' ')
         expect(body).to have_text(expected_description)
         expect(body).to have_text("DTSTART;VALUE=DATE:#{Date.tomorrow.strftime('%Y%m%d')}")
         expect(body).to have_text("URL:#{issue_url(issue)}")

spec/features/ics/group_issues_spec.rb

@@ -68,7 +68,7 @@ describe 'Group Issues Calendar Feed' do
 
         expect(body).to have_text("SUMMARY:test title (in #{project.full_path})")
         # line length for ics is 75 chars
-        expected_description = "DESCRIPTION:Find out more at #{issue_url(issue)}".insert(75, ' ')
+        expected_description = (+"DESCRIPTION:Find out more at #{issue_url(issue)}").insert(75, ' ')
         expect(body).to have_text(expected_description)
         expect(body).to have_text("DTSTART;VALUE=DATE:#{Date.tomorrow.strftime('%Y%m%d')}")
         expect(body).to have_text("URL:#{issue_url(issue)}")

spec/features/ics/project_issues_spec.rb

@@ -67,7 +67,7 @@ describe 'Project Issues Calendar Feed' do
 
         expect(body).to have_text("SUMMARY:test title (in #{project.full_path})")
         # line length for ics is 75 chars
-        expected_description = "DESCRIPTION:Find out more at #{issue_url(issue)}".insert(75, ' ')
+        expected_description = (+"DESCRIPTION:Find out more at #{issue_url(issue)}").insert(75, ' ')
         expect(body).to have_text(expected_description)
         expect(body).to have_text("DTSTART;VALUE=DATE:#{Date.tomorrow.strftime('%Y%m%d')}")
         expect(body).to have_text("URL:#{issue_url(issue)}")

spec/features/projects/wiki/user_views_wiki_page_spec.rb

@@ -131,7 +131,7 @@ describe 'User views a wiki page' do
   end
 
   context 'when page has invalid content encoding' do
-    let(:content) { 'whatever'.force_encoding('ISO-8859-1') }
+    let(:content) { (+'whatever').force_encoding('ISO-8859-1') }
 
     before do
       allow(Gitlab::EncodingHelper).to receive(:encode!).and_return(content)