Change invoice for accept numeric type.

[hooman-mirghasemi]

Now invoice accept amount as integer or float

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[khanzadimahdi]

These changes will affect multipay/payment too. could you fix it too?

[khanzadimahdi]

I merged this one, please install from master and check if any side effect exists or not.

I will write some tests for such things soon.

[hooman-mirghasemi]

I have already made this change in my project by inheriting from the class Invoice.
And I used "yek-pay" gateway for euro currency payment I didn't see any problem.

But since I did not use this repository for the Iranian banks, I can not say for sure that there is no problem and it is very good that you want to write a test for it.

Thank you

[khanzadimahdi]

If you use any international gateway (Paypal, skrill, etc.) please add them in the package too.

[khanzadimahdi]

I will release a new version after making sure there are no side effects. thanks for your contribution.