…value

PropertySet (and List) can handle arrays of undefined values
but can not handle the addition of an undefined value to an
existing array, or the addition of a good value to an
undefined array.

With this change we make the following allowances when reading
FITS headers that may have multiple keywords, some of which
may be undef:

* If a defined value is being added to a key that exists but
  is undefined, the defined value will override.
* If an undef value is being added to a key that exists but
  is has one or more defined values, the undef value will be
  dropped.

A test has been added to ensure that this is happening.