Remove union in struct mg_request_handler_in
Remove union in struct mg_request_handler_in

This seems to be better for some compilers (see #106), and may make
the code more readable.

GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver