Fix Web template parameters save

Steps to reproduce:

1. Create a Web Template document:
   • Name=SimpleTest
   • Processor=Raw Freemarker
   • Content=

         A: ${a}
         B: ${b}
     

2. Go to Template Configuration tab then to Parameters.
   Nuxeo correctly identifies two parameters in the template, "a" and "b".
3. Enter values for a and b: a="A", b="B".
4. Click Save.
5. Log out and log in back
6. Go back to SampleTest document and the in Template Configuration > Parameters

=> the values for a and b parameters were not correctly saved.