When searching by payment amount = null, no payment methods returned

[ebaranauskas]

Newest library version, when trying to search for payment methods with amount = null:
WebToPay::getPaymentMethodList(6028, null, 'EUR')
returns no payment methods at all, because it wrongly searches by amount=0. I expect that it would not use amount for filtering at all, if its value is null.

There is a problem in library internals. When searching for amount = null, it uses URL https://www.paysera.com/new/api/paymentMethods/6028/currency:EUR/amount: (mind ":" at the end of URL) which returns nothing, same as https://www.paysera.com/new/api/paymentMethods/6028/currency:EUR/amount:0 But one would expect if we are not filtering by amount, such URL should be used instead: https://www.paysera.com/new/api/paymentMethods/6028/currency:EUR , which returns all enabled payment methods ignoring amount.

Proposed pull request is here: #29