stream: only set size to total_size if client set it to 0
stream: if requested size is more than file size, set it to file size
Merge branch 'v2.26' of github.com:bioothod/elliptics-go into bioothod-v2.26

Conflicts:
	elliptics/session.go
Merge branch 'bioothod-v2.26' into 2.26