resolveImage backport (#1822)

* add resolveImage

* add groupDMChannel#setIcon + icon getter

* doc fix

* crawl no kill pls

* *whistles*

* channe

src/structures/GroupDMChannel.js

@@ -1,6 +1,7 @@
 const Channel = require('./Channel');
 const TextBasedChannel = require('./interfaces/TextBasedChannel');
 const Collection = require('../util/Collection');
+const Constants = require('../util/Constants');
 
 /*
 { type: 3,
@@ -105,6 +106,23 @@ class GroupDMChannel extends Channel {
     return this.client.users.get(this.ownerID);
   }
 
+  /**
+   * The URL to this guild's icon
+   * @type {?string}
+   * @readonly
+   */
+  get iconURL() {
+    if (!this.icon) return null;
+    return Constants.Endpoints.Channel(this).Icon(this.client.options.http.cdn, this.icon);
+  }
+
+  edit(data) {
+    const _data = {};
+    if (data.name) _data.name = data.name;
+    if (data.icon) _data.icon = data.icon;
+    return this.client.rest.methods.updateGroupDMChannel(this, _data);
+  }
+
   /**
    * Whether this channel equals another channel. It compares all properties, so for most operations
    * it is advisable to just compare `channel.id === channel2.id` as it is much faster and is often
@@ -140,6 +158,20 @@ class GroupDMChannel extends Channel {
     });
   }
 
+  /**
+   * Set a new GroupDMChannel icon.
+   * @param {Base64Resolvable|BufferResolvable} icon The new icon of the group dm
+   * @returns {Promise<Guild>}
+   * @example
+   * // Edit the group dm icon
+   * channel.setIcon('./icon.png')
+   *  .then(updated => console.log('Updated the channel icon'))
+   *  .catch(console.error);
+   */
+  setIcon(icon) {
+    return this.client.resolver.resolveImage(icon).then(data => this.edit({ icon: data }));
+  }
+
   /**
    * When concatenated with a string, this automatically concatenates the channel's name instead of the Channel object.
    * @returns {string}