check for pipe method only

See discussion here - https://github.com/form-data/form-data/pull/150#issuecomment-151884281

index.js

@@ -84,7 +84,7 @@ function asCallback(opts, cb) {
 	var ee = requestAsEventEmitter(opts);
 
 	ee.on('request', function (req) {
-		if (isStream.readable(opts.body)) {
+		if (isStream(opts.body)) {
 			opts.body.pipe(req);
 			opts.body = undefined;
 			return;