PostgreSQL, Fix change detection caused by wrong data for bytea unescaping.

This showed up when running BinaryTest#test_load_save with the more
restrictive input string handling of pg-0.18.0.pre20141117110243.gem .

Bytea values sent to the server are in binary format, but are
returned back as escaped text. To fulfill the assumption that
type_cast_from_database(type_cast_for_database(binary)) == binary
we unescape only, if the value was really received from the server.