Add spec

spec/grape/api_spec.rb

@@ -2394,7 +2394,7 @@ class ChildError < ParentError; end
     end
   end
 
-  describe '.error_format' do
+  describe '.error_formatter' do
     it 'rescues all errors and return :txt' do
       subject.rescue_from :all
       subject.format :txt
@@ -2460,6 +2460,18 @@ def self.call(message, _backtrace, _options, _env, _original_exception)
         get '/exception'
         expect(last_response.body).to eq('message: rain! @backtrace')
       end
+
+      it 'returns a modified error  with a custom error format' do
+        subject.rescue_from :all, backtrace: true do |e|
+          error!("raining dogs and cats", 418, {}, e.backtrace, e)
+        end
+        subject.error_formatter :txt, ApiSpec::CustomErrorFormatter
+        subject.get '/exception' do
+          raise 'rain!'
+        end
+        get '/exception'
+        expect(last_response.body).to eq('message: raining dogs and cats @backtrace')
+      end
     end
 
     describe 'with' do