Fix 'selectOneRadioDirectory' when used with a boolean property

In 6.0, using a vocabulary with 2 entries (IDs TRUE and FALSE) used to work with a selectOneRadioDirectory on a boolean property. Since LTS 2015, a validation error The value "TRUE" must be a boolean. is produced.

How to reproduce:

• create a Studio vocabulary with 2 entries:
  • id: TRUE
  • id: FALSE
• define a new document type in Studio with one boolean field
• in its creation layout, use a Generic widget with:
  • add the xpath of the boolean property in the Fields property
  • set Widget type property to selectOneRadioDirectory
  • set Vocabulary property to the vocabulary created above
  • set Required property to Yes
• Deploy in a Nuxeo instance
• in the Nuxeo instance, create an instance of the document type defined above with the radio button for the boolean property set to TRUE or FALSE

Expected result: the document gets created with the boolean property set to TRUE or FALSE.
Actual result: a validation error The value "TRUE" must be a boolean. or The value "FALSE" must be a boolean. is displayed under the radio button.

This is happening since LTS 2015, it was working in 6.0.