confusing behavior with `depends_on` kwarg

When using `Property`, the `depends_on` kw can be set to a string or a list. In the case where we set its value to one value, it's behavior changes based on a confusing way we handle these two cases. The code below shows a case where when a string is used the property gets updated when items change as well as the list itself, while in the second case, the property does not update when the items change.

```
from traits.api import HasPrivateTraits, List, Int,\
Property, cached_property

class TestScores ( HasPrivateTraits ):

scores = List( Int )
average = Property( depends_on = 'scores' )

@cached_property
def _get_average ( self ):
s = self.scores
return (float( reduce( lambda n1, n2: n1 + n2, s, 0 ) )
/ len( s ))

class TestScoresList ( HasPrivateTraits ):

scores = List( Int )
average = Property( depends_on = ['scores'] )

@cached_property
def _get_average ( self ):
s = self.scores
return (float( reduce( lambda n1, n2: n1 + n2, s, 0 ) )
/ len( s ))

if __name__ == '__main__':

t = TestScores(scores = [1,2,3])
print t.scores, t.average
t.scores[2] = -100
print t.scores, t.average

t2 = TestScoresList(scores=[1,2,3])
print t2.scores, t2.average
t2.scores[2] = -100
print t2.scores, t2.average
```

The root of the behavior comes from these lines in the `has_traits.py` file:

```
if isinstance( depends_on, SequenceTypes ):
depends_on = ','.join( depends_on )
else:
# Note: We add the leading blank to force it to be treated
# as using the extended trait notation so that it will
# automatically add '_items' listeners to lists/dicts:
depends_on = ' ' + depends_on
```

This should probably be changed so that both cases end up with the same behavior