fix(afs): change doc.update() parameter type to Partial<T> (#1247)

Need to follow on with doc changes + use Partial<T> in more places that make sense.
closes #1245, closes #1215

src/firestore/document/document.ts

@@ -56,7 +56,7 @@ export class AngularFirestoreDocument<T> {
    * Update some fields of a document without overwriting the entire document.
    * @param data 
    */
-  update(data: T): Promise<void> {
+  update(data: Partial<T>): Promise<void> {
     return this.ref.update(data);
   }