Enables RSpec/ExampleWording and fixes all issues

Both avoiding 'should' and repiting 'it' on the tests description
improves reading them and also makes all descriptions consistent.

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExampleWording

spec/controllers/users/registrations_controller_spec.rb

@@ -9,7 +9,7 @@ describe Users::RegistrationsController do
     end
 
     context "when username is available" do
-      it "should return true with no error message" do
+      it "returns true with no error message" do
         get :check_username, username: "available username"
 
         data = JSON.parse response.body, symbolize_names: true
@@ -19,7 +19,7 @@ describe Users::RegistrationsController do
     end
 
     context "when username is not available" do
-      it "should return false with an error message" do
+      it "returns false with an error message" do
         user = create(:user)
         get :check_username, username: user.username