Configurable UserError status codes

Various frameworks are opinionated about the status codes they return for user-errors (eg validation failed).

Status codes that are treated as UserError can now be configured;

```ruby
# config/initializers/songkick-transport.rb
Songkick::Transport.user_error_status_codes = [409, 422]
```

Eg, Rails returns 422 for "validation failed" by default.

There's also a load of diff-cruft because of trailing whitespace removal. Sorry about that!