brew-unpack: Handle new style :DATA patches

Instances of `IOPatch` created by `patch :DATA` are not affected by re-setting
the `DATA` constant of the `Formula` instance. For these patches, we iterate
through the `patchlist` and use `instance_variable_set` to attach data.

A bit hacky, but `patchlist` has no write accessors so there isn't a clean way
to modify patch contents.