Populate payment type and serialise as string

[kz412]

Add payment type enum to PaypalPaymentDetails, and serialise this property on both PaypalPaymentDetails and CreditCardPaymentDetails as a string, in line with api docs.

Currently the _type enum is serialised as an int, leading paymentDetails looking like this when serialised (note the value of 0 for _type):

"payment_details": [
--
{
"avs_result_code": "",
"credit_card_bin": "444433",
"credit_card_company": "Visa",
"credit_card_number": "XXXX-XXXX-XXXX-1111",
"cvv_result_code": "",
"_type": 0
}

Additionally, the _type property doesn't appear to exist on the PaypalPaymentDetails object. This PR modifies both payment detail objects to always supply the type, and serialise the type as a string, i.e:

"payment_details": [
--
{
"avs_result_code": "",
"credit_card_bin": "444433",
"credit_card_company": "Visa",
"credit_card_number": "XXXX-XXXX-XXXX-1111",
"cvv_result_code": "",
"_type": "credit_card"
}