Revert "Manually rolling back #167"

This reverts commit 5506735.

index.js

@@ -54,12 +54,12 @@ function mergeObject(target, source, options) {
 }
 
 function deepmerge(target, source, options) {
-	options = options || {}
-	options.arrayMerge = options.arrayMerge || defaultArrayMerge
-	options.isMergeableObject = options.isMergeableObject || defaultIsMergeableObject
-	// cloneUnlessOtherwiseSpecified is added to `options` so that custom arrayMerge()
-	// implementations can use it. The caller may not replace it.
-	options.cloneUnlessOtherwiseSpecified = cloneUnlessOtherwiseSpecified
+	options = Object.assign({
+		arrayMerge: defaultArrayMerge,
+		isMergeableObject: defaultIsMergeableObject
+	}, options, {
+		cloneUnlessOtherwiseSpecified: cloneUnlessOtherwiseSpecified
+	})
 
 	var sourceIsArray = Array.isArray(source)
 	var targetIsArray = Array.isArray(target)

test/merge.js

@@ -637,3 +637,12 @@ test('copy symbol keys in target that do exist on the target', function(t) {
 	t.equal(res[mySymbol], 'value1')
 	t.end()
 })
+
+test('should not mutate options', function(t) {
+	var options = {};
+
+	merge({}, {}, options);
+
+	t.deepEqual(options, {});
+	t.end();
+})