[FIX] expression: ensure tuple to compare leaf with TRUE/FALSE leaf

When setting a custom filter with as domain
[(0, '=', 1)]

the domain was rejected, because (0, '=', 1) wasn't
considered as a valid leaf, while it is.

This is because the Javascript converts this domain
using list instead of tuple
[(0, '=', 1)] -> [[0, '=', 1]]

And therefore, comparing the "list" leaf
to the TRUE/FALSE leaf tuple failed.

Ensuring "element" as a tuple solves the issue.

opw-640306

openerp/osv/expression.py

@@ -401,7 +401,7 @@ def is_leaf(element, internal=False):
         and len(element) == 3 \
         and element[1] in INTERNAL_OPS \
         and ((isinstance(element[0], basestring) and element[0])
-             or element in (TRUE_LEAF, FALSE_LEAF))
+             or tuple(element) in (TRUE_LEAF, FALSE_LEAF))
 
 
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