Fix HTTP methods

I'm not entirely sure how this was ever working. Potentially the HTTPoison library changed, where these methods (without the bang) returned the response object directly and then moved towards the more idiomatic tuple return with `{:ok, response}`.

Either way, the latest version of HTTPoison (0.5.0) returns the tuple version so calling `body/0` on it causes an error. Ie, this causes an error:

```elixir
{:ok, response}.body
```