Fix tests

[marioortizmanero]

I think the issue with the tests is that Rspotify had a bug for multithreaded requests, which is needed for Rust tests. The reqwest client (CLIENT) was a global with no thread safety at all (for some reason Rust didn't detect that). Thus, there are two possible solutions:

1. Keep the global client, but wrap it with an Arc<Mutex<T>> so that it's thread-safe. But global state is usually a bad idea IMO.
2. Move the client inside the Spotify struct itself. This means that it can no longer derive Serialize and Deserialize, but I don't see why these were needed anyway, and it makes sense to me to have a client for each Spotify object.

By the way, I would like to know why Serialize and Deserialize are needed for structs such as Spotify and ApiError. Shouldn't these be cleaned up? Okay I've just seen this commit b7b7c6f

This should close #95.

[ramsayleung]

Awesome! merged :)

[ramsayleung]

Learned from this case, I think we could just avoid Singleton, and make tokio::Runtime as a field of Struct Spotify ?