Add content type checks to api/v2 request specs (#31983)

spec/requests/api/v2/instance_spec.rb

@@ -15,6 +15,9 @@ RSpec.describe 'Instances' do
         expect(response)
           .to have_http_status(200)
 
+        expect(response.content_type)
+          .to start_with('application/json')
+
         expect(response.parsed_body)
           .to be_present
           .and include(title: 'Mastodon')
@@ -30,6 +33,9 @@ RSpec.describe 'Instances' do
         expect(response)
           .to have_http_status(200)
 
+        expect(response.content_type)
+          .to start_with('application/json')
+
         expect(response.parsed_body)
           .to be_present
           .and include(title: 'Mastodon')