set the session context

fixes https://github.com/ipfs/go-bitswap/issues/131

blockservice.go

@@ -115,14 +115,16 @@ func NewSession(ctx context.Context, bs BlockService) *Session {
 	exch := bs.Exchange()
 	if sessEx, ok := exch.(exchange.SessionExchange); ok {
 		return &Session{
-			ses:    nil,
-			sessEx: sessEx,
-			bs:     bs.Blockstore(),
+			sessCtx: ctx,
+			ses:     nil,
+			sessEx:  sessEx,
+			bs:      bs.Blockstore(),
 		}
 	}
 	return &Session{
-		ses: exch,
-		bs:  bs.Blockstore(),
+		ses:     exch,
+		sessCtx: ctx,
+		bs:      bs.Blockstore(),
 	}
 }
 

blockservice_test.go

@@ -102,6 +102,9 @@ type fakeSessionExchange struct {
 	session exchange.Fetcher
 }
 
-func (fe *fakeSessionExchange) NewSession(context.Context) exchange.Fetcher {
+func (fe *fakeSessionExchange) NewSession(ctx context.Context) exchange.Fetcher {
+	if ctx == nil {
+		panic("nil context")
+	}
 	return fe.session
 }