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

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

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

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