Record::buildFields() returns prematurely

[clphillips]

Since Record::buildFields() also builds keys, it should ensure that keys is also empty before returning.

Currently it's impossible to alter just an index, without also altering fields.

$record->setKey(array('my_index'), 'index', null, false)
    ->alter('my_table'); 

Should produce:

ALERT TABLE `my_table` DROP INDEX `my_index`

Actual result:

ALERT TABLE `my_table`