[IMP] field.char: make string and size optional

* Increases coherence with other fields (default string is 'unknown')
* Char fields defaut to unbounded, bounds are optional

bzr revid: xmo@openerp.com-20120723134931-b5moeenk0db5m2ss

openerp/osv/fields.py

@@ -181,7 +181,7 @@ class reference(_column):
 class char(_column):
     _type = 'char'
 
-    def __init__(self, string, size, **args):
+    def __init__(self, string="unknown", size=None, **args):
         _column.__init__(self, string=string, size=size, **args)
         self._symbol_set = (self._symbol_c, self._symbol_set_char)