SoupRequestHTTP: reorganize the async code a bit
SoupRequestHTTP: reorganize the async code a bit

Use the same "helper" data structure for all three cases (load from
SoupHTTPInputStream, load from cache immediately, and load from cache
after validating).

Distrotech/libsoup,Distrotech/libsoup,fanc999/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,fanc999/libsoup,Distrotech/libsoup,Distrotech/libsoup,fanc999/libsoup,Distrotech/libsoup