Repository.writeFile fails if no options object is passed

In the [documentation](http://github-tools.github.io/github/docs/3.1.0/Repository.html#writeFile) about `Repository.writeFile`, the `options` parameter is specified as optional. Despite this, the function fails if no `options` object is passed and throws this error:
```
TypeError: Cannot read property 'encode' of undefined
at Repository.writeFile (/Users/lucasantchougov/Documents/Work/oauth-rule-manager/node_modules/github-api/dist/components/Repository.js:941:37)
at GitHubRuleManager.commitWorkspace (/Users/lucasantchougov/Documents/Work/oauth-rule-manager/lib/rule_managers/github-rule-manager.js:143:18)
at /Users/lucasantchougov/Documents/Work/matey/matey-server/app.js:74:25
at Layer.handle [as handle_request] (/Users/lucasantchougov/Documents/Work/matey/matey-server/node_modules/express/lib/router/layer.js:95:5)
at next (/Users/lucasantchougov/Documents/Work/matey/matey-server/node_modules/express/lib/router/route.js:137:13)
at Route.dispatch (/Users/lucasantchougov/Documents/Work/matey/matey-server/node_modules/express/lib/router/route.js:112:3)
at Layer.handle [as handle_request] (/Users/lucasantchougov/Documents/Work/matey/matey-server/node_modules/express/lib/router/layer.js:95:5)
at /Users/lucasantchougov/Documents/Work/matey/matey-server/node_modules/express/lib/router/index.js:281:22
at param (/Users/lucasantchougov/Documents/Work/matey/matey-server/node_modules/express/lib/router/index.js:354:14)
at param (/Users/lucasantchougov/Documents/Work/matey/matey-server/node_modules/express/lib/router/index.js:365:14)
```

The error seems to get thrown when `options.encode` is read in `Repository.writeFile`. It only works for me when passing an empty object. This should be better documented or a default empty object should be instantiated in the method.

レポートで dist/components/Repository.js にあると特定されている Repository.writeFile から始め、リンク先の Repository ドキュメントとその動作を比較します。options オブジェクトなしで呼び出しを再現し、その後、ドキュメントに記載されたオプションのパラメーターと観測された動作が一致することを、関連するドキュメントまたは利用可能であれば対象を絞ったテストによって確認します。

索引モデルが issue の本文から書いたものです。