feat(skipLast): counts zero or less will mirror the source

BREAKING CHANGE: `skipLast` will no longer error when passed a negative number, rather it will simply return the source, as though `0` was passed.

src/internal/ReplaySubject.ts

@@ -81,7 +81,7 @@ export class ReplaySubject<T> extends Subject<T> {
       subscriber.next(copy[i] as T);
     }
 
-    this.checkFinalizedStatuses(subscriber);
+    this._checkFinalizedStatuses(subscriber);
 
     return subscription;
   }

src/internal/operators/skipLast.ts

@@ -42,6 +42,7 @@ import { OperatorSubscriber } from './OperatorSubscriber';
  * @name skipLast
  */
 export function skipLast<T>(skipCount: number): MonoTypeOperatorFunction<T> {
+  // For skipCounts less than or equal to zero, we are just mirroring the source.
   return (source: Observable<T>) => skipCount <= 0 ? source : lift(source, function (this: Subscriber<T>, source: Observable<T>) {
     const subscriber = this;
     // A ring buffer to hold the values while we wait to see