Fixed #19954 -- Fixed MySQL _last_executed decoding

Queries can contain binary data undecodable with utf-8. In this
case, using the 'replace' errors mode when decoding seems like
an acceptable representation of the query.
Thanks Marcel Ryser for the report.

Conflicts:
	django/db/backends/mysql/base.py
	tests/regressiontests/backends/tests.py
IMT: Bumped Version