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.

Start with Repository.writeFile, which the report identifies in dist/components/Repository.js, and compare its behavior with the linked Repository documentation. Reproduce the call without an options object, then verify that the documented optional parameter and the observed behavior agree, either through the relevant documentation or a focused test if one is available.

Scritto dal modello di indicizzazione a partire dal testo della issue.