Update WatirWebdriver::PageObject#nested_frames to support multiple identifiers

Watir allows frames/iframes to be located using multiple identifiers. For example:

````ruby
browser.iframe(:class => 'iframe', :name => 'frame2').exists?
````

While the page object allows multiple identifiers to be specified:

````ruby
in_iframe(:class => 'iframe', :name => 'frame2') do |frame|
  text_field(:text_field_2_multiple_identifiers, :name => 'recieverElement', :frame => frame)      
end
````

It only uses the first identifier. When locating the above element, the #nested_frame method only returns "iframe(:class => 'iframe')." instead of "iframe(:class => 'iframe', :name => 'frame2').".