[FIX] openerp: monetary not xmlrpc serializable

Since 1ba9104 monetary fields values could be represented by a
float_precision which is aimed as an improvment for digit precision when
saving a monetary field value to the database.

xmlrpclib (we are using to serialze the response) is handling some
common datatypes, but for custom one we would need to register it with
how it needs to be serialized.

This commit add a convert_to_read method to the Monetary field so when
reading monetary fields value we get the float numbers and not the
float_precision ones.

The float_precision is needed when saving, so it is alright to have float
when reading.

fixes odoo#13857
closes odoo#14125

opw-692373

openerp/fields.py

@@ -1185,6 +1185,13 @@ def convert_to_cache(self, value, record, validate=True):
                 return float_precision(value, currency.decimal_places)
         return float(value or 0.0)
 
+    def convert_to_read(self, value, record, use_name_get=True):
+        # float_precision values are not supported in pure XMLRPC
+        return float(value)
+
+    def convert_to_write(self, value, record):
+        return value
+
 
 class _String(Field):
     """ Abstract class for string fields. """