code: public Invoices getAccountInvoices(final String accountCode, final InvoiceState state, final QueryParams params) {
          if (state != null) params.put("state", state.getType());
          return doGET(Accounts.ACCOUNTS_RESOURCE + "/" + accountCode + Invoices.INVOICES_RESOURCE,
                  Invoices.class, params);
      }
docs: Lookup an account's invoices given query params
  <p>
  Returns the account's invoices
  
  @param accountCode recurly account id
  @param state {@link InvoiceState} state of the invoices
  @param params {@link QueryParams}
  @return the invoices associated with this account on success, null otherwise