Treat methodize default_from placeholders as 'no default' (i2.is_placeholder_default) #50

Follow-up of i2mint/i2#120 (PR i2mint/i2#121, once released). A methodize(..., default_from=...) signature default is an i2.wrapper.InstanceAttrDefault marker, not a JSON-able value. param_default in front/util.py only treats NotSet as "no default" (via is_not_set).

To do: use i2.is_placeholder_default (NotSet or any i2.DefaultPlaceholder) when available, keeping the is_not_set fallback for older i2; add a test feeding a methodized function.

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