Implemented the scheduler version of the 'Return' operator

Author: zsxwing

rxjava-core/src/main/java/rx/Observable.java

@@ -1012,6 +1012,31 @@ public static <T> Observable<T> just(T value) {
         return from(list);
     }
 
+    /**
+     * Returns an Observable that emits a single item and then completes in a specified scheduler.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/just.png">
+     * <p>
+     * To convert any object into an Observable that emits that object, pass that object into the
+     * <code>just</code> method.
+     * <p>
+     * This is similar to the {@link #from(java.lang.Object[])} method, except that
+     * <code>from()</code> will convert an {@link Iterable} object into an Observable that emits
+     * each of the items in the Iterable, one at a time, while the <code>just()</code> method
+     * converts an Iterable into an Observable that emits the entire Iterable as a single item.
+     *
+     * @param value
+     *            the item to pass to the {@link Observer}'s {@link Observer#onNext onNext} method
+     * @param scheduler
+     *            the scheduler to send the single element on
+     * @param <T>
+     *            the type of that item
+     * @return an Observable that emits a single item and then completes
+     */
+    public static <T> Observable<T> just(T value, Scheduler scheduler) {
+        return just(value).observeOn(scheduler);
+    }
+
     /**
      * Flattens a sequence of Observables emitted by an Observable into one Observable, without any
      * transformation.