Fix binding of ignore_filter

[pfeiffer]

This PR fixes the context in which the ignore_filter is called. This is needed when using the helper methods in a context of an eg. Rails helper.

To illustrate the difference:

Split.configure do |config|
  config.ignore_filter = ->(request) { puts "{request.class.to_s}" }
end

When calling ab_test in a controller, the output is:

<ActionDispatch::Request ...>

But when using ab_test in a helper, the output is:

<Class...>

This is especially funky due to Rails' delegation of eg. #headers, meaning that calling request.headers in the ignore_filter would have two different meanings - in helpers, it would call headers on the response, while in controllers it would call headers on the request (as intended)

This PR fixes the issue and should preserve backwards-compatibility (for methods with request as argument in the proc).

[andrew]

Thanks!