/*
build XML input for XML requests (POST).
@param {object} options: request context. see `xmlRequest()` docs.
for repeatable fields, use an array value (see below).
function buildXmlInput(options) {
  debug('buildXmlInput', options);

  var data = {}, root;
  var params = options.params || {};

  root = data[options.opType + 'Request'] = [];   // e.g. <GetOrdersRequest>

  if (options.serviceName === 'Finding') {
    root.push({'_attr': {'xmlns': 'http://www.ebay.com/marketplace/search/v1/services'}});
  }
  else {
    root.push({'_attr': {'xmlns': 'urn:ebay:apis:eBLBaseComponents'}});
  }

  if (options.authToken) {
    root.push({ 'RequesterCredentials': [ { 'eBayAuthToken' : options.authToken } ] });
  }

  // concat
  root.push.apply(root, deepToArray(params));

  debug('Converting to XML', data);

  var xmlBody =
    '<?xml version="1.0" encoding="UTF-8"?>\n' +
    xmlBuilder([data], true);

  debug('XML body', xmlBody);

  return xmlBody;
}