topcat: fix bug in reading session files containing 1-column tables.

Degeneracy in VOTable serialization of scalars and 1-element arrays
meant that topcat_session:* metadata could end up as the wrong class
and cause a ClassCastException (which was already being correctly
intercepted, so consequences not too bad).  Visible result (reported
by Marcus Hauser, ARI Heidelberg) was that session metadata like table
label got lost on session file reload.

Implemented explicit test for scalar values supplied for array
metadata to fix this.