SecurePayAU: Allow custom request_timeout

[dacook]

This method was copied from similar NabTransactGateway, but placed at the top because it contains a default value.

Note that the remote tests fix from https://github.com/activemerchant/active_merchant/pull/3979 (awaiting merge) is also included in order for tests to pass.

rake test TEST=test/unit/gateways/secure_pay_au_test.rb 25 tests, 109 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed

rake test TEST=test/remote/gateways/remote_secure_pay_au_test.rb 18 tests, 56 assertions, 0 failures, 0 errors, 0 pendings, 2 omissions, 0 notifications 100% passed