Fix Unify::find_least_abstract
Fix Unify::find_least_abstract

Only default to a variable in pre if it really is the least abstract
type.

rTreutlein/atomspace,rTreutlein/atomspace,misgeatgit/atomspace,AmeBel/atomspace,misgeatgit/atomspace,AmeBel/atomspace,rTreutlein/atomspace,misgeatgit/atomspace,misgeatgit/atomspace,AmeBel/atomspace,misgeatgit/atomspace,rTreutlein/atomspace,AmeBel/atomspace,rTreutlein/atomspace,AmeBel/atomspace