This fixes a critical bug meaning that using many_many fields in full text searching would have always failed.
the $singleton->many_many() lookup returns an array() of many-many components, however the line $class = $manyMany[0] is wrong, as the first value of the array is always the $dataClass (parentClass), not the otherClass (childClass).
Changing this to $class = $manyMany[1] fixes this bug.
Its impossible for SearchUpdater#handle_manipulation to figure out the difference
between writing to a table-less class (like Page if theres no $db set) and the
table-having parent (like SiteTree) because it only examines the DB manipulation
This meant if you tried to index Page without setting $db fields, only subclasses
that did have $db fields would be indexed
We cant fix, but we can throw an error if you try to do that
