Fix spinSome logic (osrf#49)

The workAvailable flag was never being set to true.
If I understand the original intention, I think that the flag should be set as long as there are things being executed.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

Author: jacobperron

rcljava/src/main/java/org/ros2/rcljava/executors/BaseExecutor.java

@@ -404,6 +404,7 @@ private void spinSomeImpl(long maxDurationNs, boolean exhaustive) {
       AnyExecutable anyExecutable = getNextExecutable();
       if (anyExecutable != null) {
         executeAnyExecutable(anyExecutable);
+        workAvailable = true;
       } else {
         if (!workAvailable || !exhaustive) {
           break;