Fix population of MapAnnotation.Value

The previous logic for setMapAnnotationValue was a no-op unless the
Value had already been set to non-null.  Now, as long as the
MapAnnotation object at the specified index is non-null (i.e.
setMapAnnotationID(...) has been called), the Value attribute will be
set.
Merge pull request #23 from melissalinkert/map-value-fix

Fix population of MapAnnotation.Value