handle ignoreUndefinedProperties in set(merge: true)

Previously any field in the document would be propagated into the
FieldMask regardless of whether or not it had an undefined value, which
led to the client acting as if the user had passed FieldValue.delete(),
which wasn't intended.

Fixes googleapis/nodejs-firestore#1392 for the JS SDK.

Author: wilhuff

packages/firestore/src/api/user_data_reader.ts

@@ -741,7 +741,11 @@ export function parseData(
   } else {
     // If context.path is null we are inside an array and we don't support
     // field mask paths more granular than the top-level array.
-    if (context.path) {
+    //
+    // If the input is undefined it can never participate in the fieldMask. With
+    // `ignoreUndefinedProperties` set to false, `parseScalarValue` will reject
+    // an undefined value.
+    if (context.path && input !== undefined) {
       context.fieldMask.push(context.path);
     }
 

packages/firestore/test/integration/api/fields.test.ts

@@ -398,6 +398,15 @@ apiDescribe('`undefined` properties', (persistence: boolean) => {
     });
   });
 
+  it('are ignored in set({ merge: true })', () => {
+    return withTestDocAndSettings(persistence, settings, async doc => {
+      await doc.set({ foo: 'foo', 'bar': 'unchanged' });
+      await doc.set({ foo: 'foo', 'bar': undefined }, { merge: true });
+      const docSnap = await doc.get();
+      expect(docSnap.data()).to.deep.equal({ foo: 'foo', bar: 'unchanged' });
+    });
+  });
+
   it('are ignored in update()', () => {
     return withTestDocAndSettings(persistence, settings, async doc => {
       await doc.set({});