[FIX] http: make sure to instantiate an environment before calling method

This fixes an issue in property `field.digits` that cannot find a valid cursor
to the database.  Forcing the instantiation of an environment makes the cursor
retrievable.

openerp/http.py

@@ -896,7 +896,8 @@ class Model(object):
                 raise Exception("Access denied")
             mod = request.registry[self.model]
             meth = getattr(mod, method)
-            cr = request.cr
+            # make sure to instantiate an environment
+            cr = request.env.cr
             result = meth(cr, request.uid, *args, **kw)
             # reorder read
             if method == "read":