Add spec to demonstrate usage

spec/grape/exceptions/body_parse_errors_spec.rb

@@ -91,6 +91,46 @@ def app
     end
   end
 
+  context 'api with rescue_from :grape_exceptions handler with block' do
+    subject { Class.new(Grape::API) }
+
+    before do
+      subject.rescue_from :grape_exceptions do |e|
+        rack_response "Custom Error Contents, Original Message: #{e.message}", 400
+      end
+
+      subject.params do
+        requires :beer
+      end
+
+      subject.post '/beer' do
+        'beer received'
+      end
+    end
+
+    def app
+      subject
+    end
+
+    context 'with content_type json' do
+      it 'returns body parsing error message' do
+        post '/beer', 'test', 'CONTENT_TYPE' => 'application/json'
+        expect(last_response.status).to eq 400
+        expect(last_response.body).to include 'message body does not match declared format'
+        expect(last_response.body).to include 'Custom Error Contents, Original Message'
+      end
+    end
+
+    context 'with content_type xml' do
+      it 'returns body parsing error message' do
+        post '/beer', 'test', 'CONTENT_TYPE' => 'application/xml'
+        expect(last_response.status).to eq 400
+        expect(last_response.body).to include 'message body does not match declared format'
+        expect(last_response.body).to include 'Custom Error Contents, Original Message'
+      end
+    end
+  end
+
   context 'api without a rescue handler' do
     subject { Class.new(Grape::API) }