MDL-38719 forms: detectMissingSetType() handle of repetitions.

It seems that formslib, for repetition, relies in the type
especified by the main (non existent) element, instead of
assingning a type to each repetition.

Surely a result of storing types apart from element. :-(

With the patch, detectMissingSetType() will detect
repeated elements (name containing '[') and will
perfomr the verification against that main (non existent)
element.