set the session context

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

blockservice.go

@@ -115,6 +115,7 @@ func NewSession(ctx context.Context, bs BlockService) *Session {
 	exch := bs.Exchange()
 	if sessEx, ok := exch.(exchange.SessionExchange); ok {
 		return &Session{
+			sessCtx: ctx,
 			ses:     nil,
 			sessEx:  sessEx,
 			bs:      bs.Blockstore(),
@@ -122,6 +123,7 @@ func NewSession(ctx context.Context, bs BlockService) *Session {
 	}
 	return &Session{
 		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
 }