Hyperclient::LinkCollection#include? Broken?

Behold:

```
> resource.links.to_hash.keys
=> ["self", "profile"]
> resource.links.include?("profile")
=> false
```

I think this implies some weirdness in how `Enumerable` was pulled in? Anyway, it seems like something that would be useful in a hypermedia client: checking if a link you want to follow exists.

I'll dig into this some more and probably have a PR about it unless someone beats me to it. :smile: