Fixed the default Precheck mode to use a valid class

... and prevent errors being thrown
fix callback message generation data
#117 Fix bug causing error at the moment of changing the column type
Fix readme, missing array key in alter_field example
#117 Change field type. Added extra condition to the change column type feature
Merge branch 'develop' into release/2.2.1
Merge branch 'release/2.2.1'