Handlebars.compile() does not modify "options" anymore

Fixes #1327

- This commit creates a shallow copy of the "options" passed to
  Handlebars.compile() in order to prevent modifications
- Note that "new Handlebars.Compiler().compile(..., options)" still
  modify the options object. This might change in the future, if
  anybody needs a fix for that.

spec/compiler.js

@@ -73,6 +73,18 @@ describe('compiler', function() {
     it('can pass through an empty string', function() {
       equal(Handlebars.compile('')(), '');
     });
+
+    it('should not modify the options.data property(GH-1327)', function() {
+      var options = {data: [{a: 'foo'}, {a: 'bar'}]};
+      Handlebars.compile('{{#each data}}{{@index}}:{{a}} {{/each}}', options)();
+      equal(JSON.stringify(options, 0, 2), JSON.stringify({data: [{a: 'foo'}, {a: 'bar'}]}, 0, 2));
+    });
+
+    it('should not modify the options.knownHelpers property(GH-1327)', function() {
+      var options = {knownHelpers: {}};
+      Handlebars.compile('{{#each data}}{{@index}}:{{a}} {{/each}}', options)();
+      equal(JSON.stringify(options, 0, 2), JSON.stringify({knownHelpers: {}}, 0, 2));
+    });
   });
 
   describe('#precompile', function() {