[dm-validations] Setup length auto-validation for String and Text type explicitly
[dm-validations] Setup length auto-validation for String and Text type explicitly

* Do not specify the length auto-validation for all types with a String
  primitive, because while the value may be serialized as a String, the
  value itself may be something else entirely.

emmanuel/aequitas,mbj/aequitas,datamapper/dm-validations