if the previous charset is from autodetection. Add PrevDocCharset field and use it for POST document charset resolution. r=ftang, darin sr=jst, a=adt, chofman git-svn-id: svn://10.0.0.236/branches/MOZILLA_1_0_BRANCH@127752 18797224-902f-48f8-a5cc-f745e15eee43