fix bug in operations diff

data/simple1.yaml

@@ -0,0 +1,10 @@
+info:
+  title: Tufin
+  version: 1.0.0
+openapi: 3.0.3
+paths:
+  /api/test:
+    get:
+      responses:
+        200:
+          description: OK

data/simple2.yaml

@@ -0,0 +1,10 @@
+info:
+  title: Tufin
+  version: 1.0.0
+openapi: 3.0.3
+paths:
+  /api/test:
+    post:
+      responses:
+        200:
+          description: OK

diff/diff_test.go

@@ -69,6 +69,26 @@ func TestDiff_DeletedOperation(t *testing.T) {
 		"POST")
 }
 
+func TestDiff_ModifiedOperation(t *testing.T) {
+	loader := openapi3.NewLoader()
+
+	s1, err := loader.LoadFromFile("../data/simple1.yaml")
+	require.NoError(t, err)
+
+	s2, err := loader.LoadFromFile("../data/simple2.yaml")
+	require.NoError(t, err)
+
+	d, err := diff.Get(diff.NewConfig(), s2, s1)
+	require.NoError(t, err)
+
+	require.Equal(t, &diff.OperationsDiff{
+		Added:    diff.StringList{"GET"},
+		Deleted:  diff.StringList{"POST"},
+		Modified: diff.ModifiedOperations{},
+	},
+		d.PathsDiff.Modified["/api/test"].OperationsDiff)
+}
+
 func TestAddedExtension(t *testing.T) {
 	config := diff.Config{
 		IncludeExtensions: diff.StringSet{"x-extension-test": struct{}{}},

diff/operations_diff.go

@@ -99,7 +99,7 @@ func (operationsDiff *OperationsDiff) diffOperation(config *Config, operation1,
 	}
 
 	if operation1 != nil && operation2 == nil {
-		operationsDiff.Deleted = append(operationsDiff.Added, method)
+		operationsDiff.Deleted = append(operationsDiff.Deleted, method)
 		return nil
 	}