Rather than use the MySQL specific TINYTEXT, MEDIUMTEXT and LONGTEXT
datatypes, Active Record migrations use TEXT(n) where n is the limit
specified by the developer. Unfortunately how MySQL interprets n
depends on the column's encoding so any limit above 5592405 will be
interpreted as a LONGTEXT when the encoding is UTF-8.

This commit fixes this by interpreting the limit within the adapter
and using the specific MySQL datatype as appropriate.