Fix behavior of BlockingObservable.last

It now throws an IllegalArgumentException instead of returning null if no elements are emitted.

This is based on feedback from @headinthebox confirming this expected behavior:

var xs = Observable.Range(1,0);                       		// empty sequence
int x = xs.Last().Dump();                             		// throws “sequence contains no elements”       RxJava => BlockingObservable.last()
IObservable<int> ys = xs.TakeLast(1).Dump();          // OnCompleted()                                                RxJava => Observable.takeLast(1)
IObservable<int> zs = xs.LastAsync().Dump();          // OnError(“sequence contains no elements”)    RxJava => Observable.last()

Author: benjchristensen

rxjava-core/src/main/java/rx/observables/BlockingObservable.java

@@ -178,13 +178,10 @@ public Iterator<T> getIterator() {
      * <img width="640" src="https://github.com/Netflix/RxJava/wiki/images/rx-operators/B.last.png">
      * 
      * @return the last item emitted by the source {@link Observable}
+     * @throws IllegalArgumentException if source contains no elements
      */
     public T last() {
-        T result = null;
-        for (T value : toIterable()) {
-            result = value;
-        }
-        return result;
+        return new BlockingObservable<T>(o.last()).single();
     }
 
     /**

rxjava-core/src/test/java/rx/observables/BlockingObservableTest.java

@@ -49,11 +49,10 @@ public void testLast() {
         assertEquals("three", obs.last());
     }
 
-    @Test
+    @Test(expected = IllegalArgumentException.class)
     public void testLastEmptyObservable() {
         BlockingObservable<Object> obs = BlockingObservable.from(Observable.empty());
-
-        assertNull(obs.last());
+        obs.last();
     }
 
     @Test