[#57] merge metadata when converting to s-expression instead of replacing it.

Closes #57.

Author: Yannick Scherer

src/rewrite_clj/node/meta.clj

@@ -13,7 +13,7 @@
     (let [[mta data] (node/sexprs children)]
       (assert (instance? clojure.lang.IMeta data)
               (str "cannot attach metadata to: " (pr-str data)))
-      (with-meta data (if (map? mta) mta {mta true}))))
+      (vary-meta data merge (if (map? mta) mta {mta true}))))
   (length [_]
     (+ (count prefix) (node/sum-lengths children)))
   (string [_]