Only test atomic #delete_insert for :atomic_write

Repositories that do not support :atomic_write don't need to implement
atomic delete/insert operations.

Author: Tom Johnson

lib/rdf/spec/repository.rb

@@ -38,7 +38,7 @@
 
     describe '#delete_insert' do
       it 'updates transactionally' do
-        if mutable.mutable?
+        if mutable.mutable? & mutable.supports?(:atomic_write)
           expect(mutable).to receive(:commit_transaction).and_call_original
           statement = RDF::Statement(:s, RDF::URI.new("urn:predicate:1"), :o)