add support for request kwargs in GET handler

[pawelmhm]

looking here there is comment saying:

At the moment kwargs for scrapy request are not supported in GET. 
They are supported in POST handler.

this is inconvenient, we should probably add some way to support request kwargs in GET so that there is consistent interface between POST and GET method calls.

[shadiakiki1986]

IMHO I don't believe it is necessary for the POST and GET method calls to be consistent. POST was designed to support more advanced json data structures than GET to begin with.