Use Object.keys as Object.values is not supported in node<7

Object.values was introduced in 138b101
and it is not supported in node.js until version 7, so it's a mismatch
with what's declared in the `engines` section of package.json (node >= 6).

src/stringify.ts

@@ -11,7 +11,9 @@ const actionTypes: { [key: string]: string } = {
 };
 
 const charsToEscape = new Set([
-    ...Object.values(actionTypes).filter(Boolean),
+    ...Object.keys(actionTypes)
+        .map((typeKey) => actionTypes[typeKey])
+        .filter(Boolean),
     ":",
     "[",
     "]",