fix(ts): missed readonly fix in previous PR (#386)

* changed readonly array syntax to support older tsc

`readonly T[]` is new syntax for `ReadonlyArray` which is supported in typescript older than 3.4.
As of now everything works but throws error while compiling in angular 7

* missed readonly fix in previous PR

Similar to 4.3.1 fix, to fix `readonly` with `ReadonlyArray`

src/util.ts

@@ -498,7 +498,7 @@ export function copyAndExtendArray<A, V>(
  *
  * @returns Shallow copy of arr.
  */
-export function copyArray<T>(arr: readonly T[]): T[] {
+export function copyArray<T>(arr: ReadonlyArray<T>): T[] {
   return arr.slice();
 }