Tests showing incorrect BriefDescription for link transactions

The BriefDescription subs for AddLink and DeleteLink transactions
had code to handle a link value that was a string and not a URI,
but this test shows that the method doesn't return the expected
string.
Detect non-URI value for BriefDescription on transaction links

The AddLink and DeleteLink subs in BriefDescription relied on
$URI->Resolver to return false for a non-URI value, but it
returns the base object which evaluates to true. The FromURI
call explicitly returns undef if it can't determine the URI scheme
so use it in the evaluation.
Merge branch '4.0/display-link-strings-in-transaction-brief-description' into 4.0-trunk