[Postgres] fix writing default value when primary key has varchar columns

kudos to @m-kuhn to find and solve this

Author: 3nids

src/providers/postgres/qgspostgresprovider.cpp

@@ -1872,7 +1872,7 @@ bool QgsPostgresProvider::addFeatures( QgsFeatureList &flist )
         values += delim + QStringLiteral( "$%1" ).arg( defaultValues.size() + offset );
         delim = ',';
         fieldId << idx;
-        defaultValues << defaultValue( idx ).toString();
+        defaultValues << quotedValue( defaultValue( idx ) );
       }
     }