fix typings about $push and $unshift command (#113)

Author: pawsong

index.d.ts

@@ -49,8 +49,8 @@ export type Spec<T, C extends CustomCommands<object> = never> =
   | (C extends CustomCommands<infer O> ? O : never);
 
 type ArraySpec<T, C extends CustomCommands<object>> =
-  | { $push: T }
-  | { $unshift: T }
+  | { $push: T[] }
+  | { $unshift: T[] }
   | { $splice: Array<[number] | [number, number] | [number, number, T]> }
   | { [index: string]: Spec<T, C> }; // Note that this does not type check properly if index: number.
 

typings-test.ts

@@ -207,28 +207,28 @@ update(o, {
 // $ExpectType TestObject
 update(o, {
   array: {
-    $push: "string"
+    $push: ["string"]
   }
 });
 
 // $ExpectError
 update(o, {
   array: {
-    $push: 1
+    $push: [1]
   }
 });
 
 // $ExpectType TestObject
 update(o, {
   array: {
-    $unshift: "string"
+    $unshift: ["string"]
   }
 });
 
 // $ExpectError
 update(o, {
   array: {
-    $unshift: 1
+    $unshift: [1]
   }
 });
 
@@ -493,7 +493,7 @@ update(o, {
 // $ExpectType TestObject
 update(o, {
   intersection: {
-    $unshift: "string"
+    $unshift: ["string"]
   }
 });