Fix handling of multiple double dots above root in parsing of relative URI

Attempt to fix #751.

I encountered this error through usage of Gatling on a page which in some cases produced resource URLs like the ones mentioned in the issue. Although one may argue that such URLs should not be present on a page, it poses a problem in that Gatling just appears to hang (when AHC enters its infinite loop).

Due to its usage in Gatling, I think this fix should probably be merged to the 1.9.x branch as well.