fix: fix assign function signature (#558)

* Return back to having target in assign function signature

* Change index from 1 to 0

Author: yavorona

packages/optimizely-sdk/lib/utils/fns/index.ts

@@ -18,15 +18,15 @@ import { generateUUID as uuid, keyBy as keyByUtil } from '@optimizely/js-sdk-uti
 const MAX_SAFE_INTEGER_LIMIT = Math.pow(2, 53);
 
 // eslint-disable-next-line
-export function assign(...sources: [any]): any {
-  if (sources.length < 1) {
+export function assign(target: any, ...sources: any[]): any {
+  if (!target) {
     return {};
   }
   if (typeof Object.assign === 'function') {
-    return Object.assign(...sources);
+    return Object.assign(target, ...sources);
   } else {
-    const to = Object(sources[0]);
-    for (let index = 1; index < sources.length; index++) {
+    const to = Object(target);
+    for (let index = 0; index < sources.length; index++) {
       const nextSource = sources[index];
       if (nextSource !== null && nextSource !== undefined) {
         for (const nextKey in nextSource) {