add Observable.startWith(Observable) method and unit test

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

@@ -6431,6 +6431,22 @@ public final Observable<T> skipWhileWithIndex(Func2<? super T, Integer, Boolean>
         return create(OperationSkipWhile.skipWhileWithIndex(this, predicate));
     }
 
+    /**
+     * Returns an Observable that emits the items in a specified {@link Observable} before it begins to emit items
+     * emitted by the source Observable. 
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/startWith.png">
+     * 
+     * @param values
+     *            an Observable that contains the items you want the modified Observable to emit first
+     * @return an Observable that emits the items in the specified {@link Observable} and then emits the items
+     *         emitted by the source Observable
+     * @see <a href="https://github.com/Netflix/RxJava/wiki/Combining-Observables#wiki-startwith">RxJava Wiki: startWith()</a>
+     */
+    public final Observable<T> startWith(Observable<T> values) {
+        return concat(values, this);
+    }
+
     /**
      * Returns an Observable that emits the items in a specified {@link Iterable} before it begins to emit items
      * emitted by the source Observable.

rxjava-core/src/test/java/rx/StartWithTests.java

@@ -45,4 +45,17 @@ public void startWithIterable() {
         assertEquals("two", values.get(3));
     }
 
+	@Test
+    public void startWithObservable() {
+        List<String> li = new ArrayList<String>();
+        li.add("alpha");
+        li.add("beta");
+        List<String> values = Observable.from("one", "two").startWith(Observable.from(li)).toList().toBlockingObservable().single();
+
+        assertEquals("alpha", values.get(0));
+        assertEquals("beta", values.get(1));
+        assertEquals("one", values.get(2));
+        assertEquals("two", values.get(3));
+    }
+
 }