Fix MIME spec examples to not collide with actual registry

[straight-shoota]

Fixes #8793

Prefixing the file extensions should make sure they don't match any existing values in the system's MIME registry.

[DanielDavis5]

Granted that name will probably never conflict, wouldn't it just be better to reset and reinit with the loaddefaults set to false?

[straight-shoota]

I'm sure there was a reason why I didn't do that in the first place. But I don't recall. It looks you might be right.

[straight-shoota]

Updated the specs to call MIME.reset and MIME.init(load_defaults: false). Also added that to all other specs.

[Blacksmoke16]

Might be a good usecase for before_each hook now that those exist?

[RX14]

around_each would abstract the whole thing with no changes to specs, I think.

[RX14]

@straight-shoota want to work on this or should this be merged without any changes to get the specs suite fixed?

[straight-shoota]

Rebased on master and changed to use spec hooks.

[Sija]

Why not use around_each instead?

[straight-shoota]

Does it make a difference? around_each is for when you need to share state between setup and teardown steps. There's no locally shared state here, so can just use before and after hooks.