[#176] shard: Fix BlobStor access when data range is zero

In previous implementation Shard accessed the BlobStor to get the
object header. However, the shard must take headers from the metabase.
From now zero length of the requested payload range seens as object
header request. In this case shard calls metabase to get the header.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/shard/get.go

@@ -49,6 +49,9 @@ func (p *GetPrm) WithFullRange() *GetPrm {
 //
 // Calling with negative length is equivalent
 // to getting the full payload range.
+//
+// Calling with zero length is equivalent
+// to getting the object header.
 func (p *GetPrm) WithRange(off uint64, ln int64) *GetPrm {
 	if p != nil {
 		p.off, p.ln = off, ln
@@ -89,6 +92,15 @@ func (s *Shard) Get(prm *GetPrm) (*GetRes, error) {
 		return &GetRes{
 			obj: res.Object(),
 		}, nil
+	} else if prm.ln == 0 {
+		head, err := s.metaBase.Get(prm.addr)
+		if err != nil {
+			return nil, err
+		}
+
+		return &GetRes{
+			obj: head,
+		}, nil
 	}
 
 	// try to read from WriteCache