fix getTargetUrl

[Daniyal-Javani]

Due to not using redirect() helper method the getTargetUrl no longer working.

Motivation and context

related to shetabit/payment#29

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Please let me know if the PR needs any changes.

[khanzadimahdi]

why do we need this method? you can run toJson method and retrieve redirection form in JSON format.
as you know we sometimes we redirect to bank page using POST method, so getTargetUrl is not suitable here.
it's better to get JSON format and send the suitable POST or GET request according to it.

[khanzadimahdi]

you could use use render method to generate redirection form (i think you use laravel)

// in laravel controller, the below code renders a redirection form and pass it as Laravel's view
return $purchasedPayment->pay()->render();

for restful API you could use toJson method too

[khanzadimahdi]

please continue this pull request if you think this is a new feature or some bugs has resolved in your commits. thanks for your contributions.

[Daniyal-Javani]

Thanks, I haven't noticed the toJson method.
Thanks again for updating shetabit/payment#29