feat(range): accept one argument (#4360)

spec/observables/range-spec.ts

@@ -73,6 +73,21 @@ describe('range', () => {
     });
 
   });
+
+  it('should accept only one argument where count is argument and start is zero', () => {
+    const e1 = range(5)
+      .pipe(concatMap((x, i) => of(x).pipe(delay(i === 0 ? 0 : 20, rxTestScheduler))));
+    const expected = 'a-b-c-d-(e|)';
+    const values = {
+      a: 0,
+      b: 1,
+      c: 2,
+      d: 3,
+      e: 4
+    };
+    expectObservable(e1).toBe(expected, values);
+    expectObservable(e1).toBe(expected, values);
+  });
 });
 
 describe('RangeObservable', () => {

src/internal/observable/range.ts

@@ -26,7 +26,7 @@ import { Observable } from '../Observable';
  * @see {@link index/interval}
  *
  * @param {number} [start=0] The value of the first integer in the sequence.
- * @param {number} [count=0] The number of sequential integers to generate.
+ * @param {number} count The number of sequential integers to generate.
  * @param {SchedulerLike} [scheduler] A {@link SchedulerLike} to use for scheduling
  * the emissions of the notifications.
  * @return {Observable} An Observable of numbers that emits a finite range of
@@ -36,9 +36,14 @@ import { Observable } from '../Observable';
  * @owner Observable
  */
 export function range(start: number = 0,
-                      count: number = 0,
+                      count?: number,
                       scheduler?: SchedulerLike): Observable<number> {
   return new Observable<number>(subscriber => {
+    if (count === undefined) {
+      count = start;
+      start = 0;
+    }
+
     let index = 0;
     let current = start;