resolveImage backport (#1822)

* add resolveImage

* add groupDMChannel#setIcon + icon getter

* doc fix

* crawl no kill pls

* *whistles*

* channe

src/client/ClientDataResolver.js

@@ -174,6 +174,20 @@ class ClientDataResolver {
     return String(data);
   }
 
+
+  /**
+   * Resolves a Base64Resolvable, a string, or a BufferResolvable to a Base 64 image.
+   * @param {string|BufferResolvable|Base64Resolvable} image The image to be resolved
+   * @returns {Promise<string>}
+   */
+  resolveImage(image) {
+    if (!image) return Promise.resolve(null);
+    if (typeof image === 'string' && image.startsWith('data:')) {
+      return Promise.resolve(image);
+    }
+    return this.resolveFile(image).then(this.resolveBase64);
+  }
+
   /**
    * Data that resolves to give a Base64 string, typically for image uploading. This can be:
    * * A Buffer
@@ -192,19 +206,25 @@ class ClientDataResolver {
   }
 
   /**
-   * Data that can be resolved to give a Buffer. This can be:
-   * * A Buffer
-   * * The path to a local file
-   * * A URL
-   * @typedef {string|Buffer} BufferResolvable
-   */
+    * Data that can be resolved to give a Buffer. This can be:
+    * * A Buffer
+    * * The path to a local file
+    * * A URL
+    * * A Stream
+    * @typedef {string|Buffer} BufferResolvable
+    */
 
   /**
-   * Resolves a BufferResolvable to a Buffer.
-   * @param {BufferResolvable} resource The buffer resolvable to resolve
-   * @returns {Promise<Buffer>}
-   */
-  resolveBuffer(resource) {
+    * @external Stream
+    * @see {@link https://nodejs.org/api/stream.html}
+    */
+
+  /**
+    * Resolves a BufferResolvable to a Buffer.
+    * @param {BufferResolvable|Stream} resource The buffer or stream resolvable to resolve
+    * @returns {Promise<Buffer>}
+    */
+  resolveFile(resource) {
     if (resource instanceof Buffer) return Promise.resolve(resource);
     if (this.client.browser && resource instanceof ArrayBuffer) return Promise.resolve(convertToBuffer(resource));
 
@@ -229,38 +249,18 @@ class ClientDataResolver {
           });
         }
       });
+    } else if (resource.pipe && typeof resource.pipe === 'function') {
+      return new Promise((resolve, reject) => {
+        const buffers = [];
+        resource.once('error', reject);
+        resource.on('data', data => buffers.push(data));
+        resource.once('end', () => resolve(Buffer.concat(buffers)));
+      });
     }
 
     return Promise.reject(new TypeError('The resource must be a string or Buffer.'));
   }
 
-  /**
-   * @external Stream
-   * @see {@link https://nodejs.org/api/stream.html}
-   */
-
-  /**
-   * Converts a Stream to a Buffer.
-   * @param {Stream} resource The stream to convert
-   * @returns {Promise<Buffer>}
-   */
-  resolveFile(resource) {
-    return resource ? this.resolveBuffer(resource)
-      .catch(() => {
-        if (resource.pipe && typeof resource.pipe === 'function') {
-          return new Promise((resolve, reject) => {
-            const buffers = [];
-            resource.once('error', reject);
-            resource.on('data', data => buffers.push(data));
-            resource.once('end', () => resolve(Buffer.concat(buffers)));
-          });
-        } else {
-          throw new TypeError('The resource must be a string, Buffer or a valid file stream.');
-        }
-      }) :
-      Promise.reject(new TypeError('The resource must be a string, Buffer or a valid file stream.'));
-  }
-
   /**
    * Data that can be resolved to give an emoji identifier. This can be:
    * * The unicode representation of an emoji