Fix valid_response_type? discarding the fail! Rack response

[cnorthwood]

Problem

valid_response_type? calls fail!, which invokes on_failure and produces a Rack redirect response, but then discards it by returning false:

def valid_response_type?
  return true if params.key?(configured_response_type)

  error_attrs = RESPONSE_TYPE_EXCEPTIONS[configured_response_type]
  fail!(error_attrs[:key], error_attrs[:exception_class].new(params['error']))

  false  # Rack response from fail! is discarded
end

callback_phase then does return unless valid_response_type?, which returns nil. This propagates as a nil Rack response and crashes downstream middleware:

NoMethodError: undefined method '[]' for nil
  rack/content_length.rb:23:in 'Rack::ContentLength#call'

This is triggered when the OAuth provider returns an error response (e.g. ?error=access_denied) without a code param. Related to #105.

Fix

Raise the exception instead of calling fail! directly. The exception propagates to the rescue StandardError in OmniAuth::Strategy#call!, which calls fail! and properly returns the redirect response to the client.

Fixes #217