Encode href params before substituting them into the url

@greysteil This fixes https://exceptions.gocardless.com/production/gocardless-live/group/5303/

Review?

Next steps:

1. Bump Atum
2. Update Atum dependency in GCER
Not saying you should write this differently (in fact, you totally should not), but purely for a "oh this is cool, out of interest", you could write this like so:

```rb
params.map!(&URI.method(:escape))
```

Was curious to see if it would work and apparently it does. Yay Ruby.