Do not modify |options| passed to |PEG.buildParser|

Modifying |options| can lead to subtle bugs.

src/compiler/passes/generate-code.js

@@ -1,12 +1,10 @@
 /* Generates the parser code. */
 PEG.compiler.passes.generateCode = function(ast, options) {
-  options = options || {};
-  if (options.cache === undefined) {
-    options.cache = false;
-  }
-  if (options.trackLineAndColumn === undefined) {
-    options.trackLineAndColumn = false;
-  }
+  options = clone(options) || {};
+  defaults(options, {
+    cache:              false,
+    trackLineAndColumn: false
+  });
 
   /*
    * Codie 1.1.0

src/utils.js

@@ -71,6 +71,22 @@ function values(object) {
   return result;
 }
 
+function clone(object) {
+  var result = {};
+  for (var key in object) {
+    result[key] = object[key];
+  }
+  return result;
+}
+
+function defaults(object, defaults) {
+  for (var key in defaults) {
+    if (object[key] === undefined) {
+      object[key] = defaults[key];
+    }
+  }
+}
+
 /*
  * Returns a string padded on the left to a desired length with a character.
  *