get_cookies
Description:
Retrieves (in Cookie-header form) the list of cookies that would be sent with a request to uri.
If for_http is true, the return value will include cookies marked "HttpOnly" (that is, cookies 
      that the server wishes to keep hidden from client-side scripting operations such as the JavaScript document.cookies property). Since 
      CookieJar sets the Cookie header itself when making the actual HTTP request, you 
      should almost certainly be setting for_http to false if you are calling this.
Parameters:
| this | |
| uri | a Uri | 
| for_http | whether or not the return value is being passed directly to an HTTP operation | 
Returns:
| the cookies, in string form, or null if  there are no cookies for  |