[FIX] stock: correctly compute custom search method

The search on qty_available was refactored twice:
- b9853db8 to split the method
- a989680b to remove the eval operator

At b9853db8 the order of evaluation introduced a bug so that the domain
   ('qty_available', '=' 10.0)
was evaluated
   if product[qty_available] = 10.0:
which crashed (`=` instead of `==`).

Before b9853db8, this code did not crash as the operator was converted via
  if operator == '=':
    operator = '=='
which is no longer executed after the split

a989680b replaced the eval by a dictionnary that replaced the eval error by a
KeyError when searching with `=` operator.

This commit changes several things:
- replace the `operator in ('==', '>=', '<=')` as this was evaluated before the
  operator replacement code mentionned above
- remove the replacement code as no longer justified since a989680b
- replace the OPERATOR key `==` to always match given key

addons/stock/models/product.py

@@ -12,7 +12,7 @@ OPERATORS = {
     '>': py_operator.gt,
     '<=': py_operator.le,
     '>=': py_operator.ge,
-    '==': py_operator.eq,
+    '=': py_operator.eq,
     '!=': py_operator.ne
 }
 
@@ -240,9 +240,6 @@ class Product(models.Model):
         if not isinstance(value, (float, int)):
             raise UserError('Invalid domain right operand')
 
-        if operator == '=':
-            operator = '=='
-
         # TODO: Still optimization possible when searching virtual quantities
         ids = []
         for product in self.search([]):
@@ -252,7 +249,7 @@ class Product(models.Model):
 
     def _search_qty_available(self, operator, value):
         # TDE FIXME: should probably clean the search methods
-        if value == 0.0 and operator in ('==', '>=', '<='):
+        if value == 0.0 and operator in ('=', '>=', '<='):
             return self._search_product_quantity(operator, value, 'qty_available')
         product_ids = self._search_qty_available_new(operator, value, self._context.get('lot_id'), self._context.get('owner_id'), self._context.get('package_id'))
         return [('id', 'in', product_ids)]