Preserve multi-value dict items on page preview
Preserve multi-value dict items on page preview

Fixes #3564. Convert QueryDict to a plain dict when writing to session and vice versa when retrieving, so that multi-value fields are preserved.

takeflight/wagtail,gasman/wagtail,torchbox/wagtail,nealtodd/wagtail,timorieber/wagtail,rsalmaso/wagtail,gasman/wagtail,iansprice/wagtail,nimasmi/wagtail,mixxorz/wagtail,mixxorz/wagtail,takeflight/wagtail,mikedingjan/wagtail,FlipperPA/wagtail,jnns/wagtail,nimasmi/wagtail,timorieber/wagtail,mixxorz/wagtail,torchbox/wagtail,zerolab/wagtail,zerolab/wagtail,gasman/wagtail,thenewguy/wagtail,kaedroho/wagtail,mikedingjan/wagtail,rsalmaso/wagtail,zerolab/wagtail,torchbox/wagtail,nealtodd/wagtail,iansprice/wagtail,timorieber/wagtail,nealtodd/wagtail,rsalmaso/wagtail,mikedingjan/wagtail,kaedroho/wagtail,wagtail/wagtail,iansprice/wagtail,FlipperPA/wagtail,mikedingjan/wagtail,jnns/wagtail,kaedroho/wagtail,nimasmi/wagtail,takeflight/wagtail,mixxorz/wagtail,gasman/wagtail,wagtail/wagtail,jnns/wagtail,gasman/wagtail,takeflight/wagtail,nealtodd/wagtail,thenewguy/wagtail,jnns/wagtail,zerolab/wagtail,mixxorz/wagtail,FlipperPA/wagtail,zerolab/wagtail,timorieber/wagtail,rsalmaso/wagtail,thenewguy/wagtail,thenewguy/wagtail,thenewguy/wagtail,FlipperPA/wagtail,rsalmaso/wagtail,wagtail/wagtail,wagtail/wagtail,kaedroho/wagtail,kaedroho/wagtail,wagtail/wagtail,iansprice/wagtail,nimasmi/wagtail,torchbox/wagtail