Properties.Merge: check for nil (#66)

properties.go

@@ -26,6 +26,10 @@ func (p Properties) Set(name string, value interface{}) Properties {
 // Merge adds the properties from the provided `props` into the receiver `p`.
 // If a property in `props` already exists in `p`, its value will be overwritten.
 func (p Properties) Merge(props Properties) Properties {
+	if props == nil {
+		return p
+	}
+
 	for k, v := range props {
 		p[k] = v
 	}

properties_test.go

@@ -44,7 +44,18 @@ func TestPropertiesMerge(t *testing.T) {
 
 	expected := Properties{"title": "A", "value": 0.5, "currency": "USD", "service": "api"}
 
-	if !reflect.DeepEqual(props, Properties{"title": "A", "value": 0.5, "currency": "USD", "service": "api"}) {
+	if !reflect.DeepEqual(props, expected) {
+		t.Errorf("invalid properties produced by merge:\n- expected %#v\n- found: %#v", expected, props)
+	}
+}
+
+func TestPropertiesMergeNil(t *testing.T) {
+	props := NewProperties().Set("title", "A")
+	props.Merge(nil)
+
+	expected := Properties{"title": "A"}
+
+	if !reflect.DeepEqual(props, expected) {
 		t.Errorf("invalid properties produced by merge:\n- expected %#v\n- found: %#v", expected, props)
 	}
 }