Fix pseudo unique *(e.g. non-upmixed but unique)* `@icon` key to use **last value** only

This is from GM standards... GM **always** picks last value if there is a conflict to resolve... **not** the first. If we want a specific custom 45px size icon we're going to have to implement one *(for metadata blocks)* and probably/optionally put in the prefix support portion from CI.

Refs:
* https://github.com/OpenUserJs/OpenUserJS.org/pull/129#issuecomment-45696640

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Tested in dev ~~as much as can at the moment since script pages are currently offline~~ ... currently shows properly with this pr in [my dev script list](http://localhost:8080/users/Marti/scripts) with uso - More Pages and [my pro script list with this current bug](https://openuserjs.org/users/Marti/scripts). Trying not to have the same/similar mistakes from USO... let's not repeat them please. :)