use Kernel#Array to cast values to an Array

[no-reply]

squash out error prone local logic for array casting, use Kernel#Array
instead.

the older logic would fail to cast in the case that the value was a String,
requiring a tight integration between the forms input objects and the
application's forms/models. since the very next line is going to fail if we fail
to cast to an Array, let's do so aggressively here.

it looks as though this line once used Array#wrap for this, but this only
works if the object responds to #to_ary (Ruby's implicit cast); otherwise
it results in a value like [["blah"]]. Kernel#Array is willing to perform an
explicit cast.