URL-Encode request path

[MineBartekSA]

The request path should be URL-Encoded.
This change allows writing code like:

client.create_reaction(<channel id>, <message id>, "⏭")

Now nobody has to keep track of emojis and such.

[greenbigfrog]

lgtm otherwise 👍

[z64]

I'm not sure this should happen at our base request method. It will be redundant in 99% of cases - emoji are the exception where we need to assert that the content is encoded correctly - so I think it might be better to localize this to the relevant REST methods to optimize that path in the simplest way.

[MineBartekSA]

I've only added URI.encode to the relevant request paths.