[core] MySlq migration issue as BLOB/TEXT column can't have a default value

BLOB/TEXT column can't have a default value when migrating in MySQL caused by "ALTER COLUMN %s SET DEFAULT %%s"

Recent change in MySQL http://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-9.html "A memory leak occurred for attempts to use ALTER TABLE to set a default value for a tiny, medium, or long BLOB or TEXT column. (Bug #14756206)" MySQL 5.6.9 onwards because "BLOB and TEXT columns cannot have DEFAULT values." http://dev.mysql.com/doc/refman/5.0/en/blob.html

Causing MySQL Hive migration issues such as:

raise errorclass, errorvalue
_mysql_exceptions.OperationalError: (1101, "BLOB/TEXT column 'job_properties' can't have a default value")

Attached proposed patch for south/db/generic.py