Changed extract_limit in class Column to return correct mysql float and ...

...double limits

Fixes https://github.com/rails/rails/issues/14135.

When rails uses a float type with a limit in a mysql database, rails converts limits from 25..53 to double types to comply with mysql, but the rails schema still said float with no limit, causing loss of accuracy. This pull request causes floats and doubles to automatically set their limits to the mysql limits for those types (24 and 53, respectively).

There still may be an inconsistency; as shown in test_float_limits, rails automatically converts floats with limits <= 24 into floats and floats with limits >= 25 into doubles. However, according to the MySQL documentation, floats should have a precision of <= 23 and doubles should be >= 24. See MySQL documentation: http://dev.mysql.com/doc/refman/5.7/en/floating-point-types.html