Enabled collection param for Post APIs

This will allow us to browse posts based on their collection which will
be used for rendering the collections card. As with the Collections API
we're not opening up the write endpoints yet.

ghost/posts-service/lib/PostsService.js

@@ -43,7 +43,7 @@ class PostsService {
      */
     async browsePosts(options) {
         let posts;
-        if (this.isSet('collections') && options.collection) {
+        if (options.collection) {
             let collection = await this.collectionsService.getById(options.collection);
 
             if (!collection) {