Merge pull request #446 from thaJeztah/19.03_backport_ctx_upload_cancel

[19.03 backport] builder-next: close build context upload on cancel

builder/builder-next/reqbodyhandler.go

@@ -35,6 +35,7 @@ func (h *reqBodyHandler) newRequest(rc io.ReadCloser) (string, func()) {
 		h.mu.Lock()
 		delete(h.requests, id)
 		h.mu.Unlock()
+		rc.Close()
 	}
 }