do not set undefined header (#3925)

Signed-off-by: Matteo Collina <[email protected]>

Author: mcollina

lib/interceptor/cache.js

@@ -225,8 +225,11 @@ function handleResult (
 
     let headers = {
       ...opts.headers,
-      'if-modified-since': new Date(result.cachedAt).toUTCString(),
-      'if-none-match': result.etag
+      'if-modified-since': new Date(result.cachedAt).toUTCString()
+    }
+
+    if (result.etag) {
+      headers.etag = result.etag
     }
 
     if (result.vary) {