MapConstraint breaks for options.value containing "length"

[QuentinRoy]

MapConstraint breaks when created with a value parameter containing "length".
The reason is that it uses the each function to create the options.keys and options.values arrays from options.value, but each treats any object containing a "length" property as an array.
As a result option.keys ends up with integers ranging from 0 to options.value["length"] and option.values is filled up of undefined.