Allow array params

[carvil]

This change allows one to setup array params in methods such as find_items_advanced, which would enable using itemFilter as described here.

Here is a concrete example. I want to search for items in a given category, but I only want to see top rated sellers and sellers with a business type of Business. The URL would be (replace app-id with your app id):

http://svcs.ebay.com/services/search/FindingService/v1?OPERATION-NAME=findItemsAdvanced&SERVICE-VERSION=1.0.0&SECURITY-APPNAME=APP-ID&RESPONSE-DATA-FORMAT=JSON&RESTPAYLOAD=true&categoryId=267&itemFilter(0).name=SellerBusinessType&itemFilter(0).value=Business&itemFilter(1).name=TopRatedSellerOnly&itemFilter(1).value=true

Note itemFilter(0) and itemFilter(1), which indicate an array of values. With the change I've introduced, one can do the following:

response = finder.find_items_advanced({
  categoryId: 267,
  outputSelector: 'SellerInfo',
  itemFilter: [{
    name: 'TopRatedSellerOnly',
    value: 'true'
  },{
    name: 'SellerBusinessType',
    value: 'Business'
  }]
})

What do you think about this?