`invoice.list` and `invoice.get` only contains the first line of the invoice

[pcreux]

FreshBooks API returns invoices with the following format:

<invoice>
  <lines>
    <line>
      <line_id>123</line_id>
    </line>
    <line>
      <line_id>234</line_id>
    </line>
  </lines>
</invoice>

HTTParty turns this into the hash:

{ invoice: { lines: { line_id: 123 } } }