Update PropertySourcesPropertyValues so that collection values are only
added from a single PropertySource. Prior to this commit, given the
following:

    PropertySource-A
     list[0]=x

    PropertySource-B
     list[0]=y
     list[1]=z

PropertySourcesPropertyValues would take `x` from A and `z` from B,
resulting in a binding of `[x,z]`. The updated code now returns the
more logical `[x]`.

Fixes gh-2611