fix(database): allow null values for equalTo, etc.

Values passed for equalTo, startAt and endAt should be allowed to be
null.

Closes #704.

Author: cartant

src/database/firebase_list_factory.ts

@@ -49,10 +49,10 @@ export function FirebaseListFactory (
       }
 
       // check equalTo
-      if (!utils.isNil(query.equalTo)) {
-          queried = queried.equalTo(query.equalTo);
+      if (utils.hasKey(query, "equalTo")) {
+        queried = queried.equalTo(query.equalTo);
 
-        if (!utils.isNil(query.startAt) || query.endAt) {
+        if (utils.hasKey(query, "startAt") || utils.hasKey(query, "endAt")) {
           throw new Error('Query Error: Cannot use startAt or endAt with equalTo.');
         }
 
@@ -69,11 +69,11 @@ export function FirebaseListFactory (
       }
 
       // check startAt
-      if (!utils.isNil(query.startAt)) {
+      if (utils.hasKey(query, "startAt")) {
           queried = queried.startAt(query.startAt);
       }
 
-      if (!utils.isNil(query.endAt)) {
+      if (utils.hasKey(query, "endAt")) {
           queried = queried.endAt(query.endAt);
       }
 

src/utils.ts

@@ -8,6 +8,10 @@ export function isNil(obj: any): boolean {
   return obj === undefined || obj === null;
 }
 
+export function hasKey(obj: Object, key: string): boolean {
+  return obj && obj[key] !== undefined;
+}
+
 export function isString(value: any): boolean {
   return typeof value === 'string';
 }