Allow a custom request_class and params_method on Rack::Instrumentation

The reasoning for this is to filter params before sending to Appsignal.
F.E.

```
use Appsignal::Rack::Instrumentation,
  request_class: ActionDispatch::Request,
  params_method: :filtered_parameters
```
Merge pull request #24 from filipegiusti/flexibilize-params-method-on-rack-instrumentation

Allow a custom request_class and params_method on Rack::Instrumentation