Fix the field() method in LiftScreen that uses an underlying field so it...

... incorporates passes in validations as well as the validations on the underlying field. Previously, validations passed were ignored:

val username = field(User.username, valMinLen(2, "Your username is too short. Please go die (or make it longer)."))

Now they aren't ignored.

https://groups.google.com/forum/?fromgroups=#!topic/liftweb/QceKSZ2_WIA