Fix regression when generating `data` URI in redirect engine

Related feedback:
- https://www.reddit.com/r/uBlockOrigin/comments/cpxm1v/

Put back erroneously removed code which enable to
generate a `data` URI from already encoded resources.

src/js/cachestorage.js

@@ -56,7 +56,7 @@
 
     // Default to webext storage. Wrapped into promises if the API does not
     // support returning promises.
-    const promisified = (function() {
+    const promisified = (( ) => {
         try {
             return browser.storage.local.get('_') instanceof Promise;
         }
@@ -394,19 +394,19 @@
             if ( keys.length === 0 ) { return callback(); }
             getDb().then(db => {
                 if ( !db ) { return callback(); }
-                let finish = ( ) => {
+                const finish = ( ) => {
                     if ( callback === undefined ) { return; }
                     let cb = callback;
                     callback = undefined;
                     cb();
                 };
                 try {
-                    let transaction = db.transaction(STORAGE_NAME, 'readwrite');
+                    const transaction = db.transaction(STORAGE_NAME, 'readwrite');
                     transaction.oncomplete =
                     transaction.onerror =
                     transaction.onabort = finish;
-                    let table = transaction.objectStore(STORAGE_NAME);
-                    for ( let key of keys ) {
+                    const table = transaction.objectStore(STORAGE_NAME);
+                    for ( const key of keys ) {
                         table.delete(key);
                     }
                 } catch (ex) {
@@ -420,7 +420,7 @@
                 callback = noopfn;
             }
             getDb().then(db => {
-                let transaction = db.transaction(STORAGE_NAME, 'readwrite');
+                const transaction = db.transaction(STORAGE_NAME, 'readwrite');
                 transaction.oncomplete =
                 transaction.onerror =
                 transaction.onabort = ( ) => {

src/js/redirect-engine.js

@@ -201,6 +201,8 @@ const RedirectEntry = class {
     // cause leakage of extension id. See:
     // - https://stackoverflow.com/a/8056313
     // - https://bugzilla.mozilla.org/show_bug.cgi?id=998076
+    // https://www.reddit.com/r/uBlockOrigin/comments/cpxm1v/
+    //   User-supplied resources may already be base64 encoded.
 
     toURL(fctxt, asDataURI = false) {
         if (
@@ -219,7 +221,11 @@ const RedirectEntry = class {
             return `data:${mime},`;
         }
         if ( this.data.startsWith('data:') === false ) {
+            if ( this.mime.indexOf(';') === -1 ) {
                 this.data = `data:${this.mime};base64,${btoa(this.data)}`;
+            } else {
+                this.data = `data:${this.mime},${this.data}`;
+            }
         }
         return this.data;
     }