- Fixed unicode support for PyMySQL when using an "executemany"

- Fixed unicode support for PyMySQL when using an "executemany"
operation with unicode parameters.  SQLAlchemy now passes both
the statement as well as the bound parameters as unicode
objects, as PyMySQL generally uses string interpolation
internally to produce the final statement, and in the case of
executemany does the "encode" step only on the final statement.
fixes #3337

(cherry picked from commit dcf5408f7d315b4d9ddec5d0d696eb364d763099)

Conflicts:
	lib/sqlalchemy/dialects/mysql/pymysql.py