Slice filter: prevented slice redirection (ticket #1219).
Slice filter: prevented slice redirection (ticket #1219).

When a slice subrequest was redirected to a new location, its context was lost.
After its completion, a new slice subrequest for the same slice was created.
This could lead to infinite loop.  Now the slice module makes sure each slice
subrequest starts output with the slice context available.

hy0kl/nginx,hy0kl/nginx,hy0kl/nginx