Don't overwrite settings with defaults

Also changes the way the JSON fields are set to force ActiveRecord
to recognise the changes. Is there a better way?

lib/travis/api/v3/models/settings.rb

@@ -11,8 +11,10 @@ module Travis::API::V3
     end
 
     def update(settings = {})
-      settings = defaults.merge(settings)
-      repository.update_attributes(settings: JSON.generate(settings))
+      settings = to_h.merge(settings)
+      repository.settings.clear
+      settings.each { |k, v| repository.settings[k] = v }
+      repository.save!
     end
 
     private

spec/v3/services/settings_spec.rb

@@ -107,6 +107,7 @@ describe Travis::API::V3::Services::Settings do
       let(:params) { JSON.dump('settings.build_pushes' => false) }
 
       before do
+        repo.update_attributes(settings: JSON.dump('maximum_number_of_builds' => 20))
         patch("/v3/repo/#{repo.id}/settings", params, json_headers.merge(auth_headers))
       end
 
@@ -118,7 +119,7 @@ describe Travis::API::V3::Services::Settings do
             'builds_only_with_travis_yml' => false,
             'build_pushes' => false,
             'build_pull_requests' => true,
-            'maximum_number_of_builds' => 0
+            'maximum_number_of_builds' => 20
           }
         )
       end