Repository.writeFile not working

When attempting to use Repository.writeFile, this error occurs:
```
(node:8240) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'status' of undefined
at handler (C:\Program Files\nodejs\node_modules\github-api\dist\components\Requestable.js:372:40)
at processTicksAndRejections (internal/process/task_queues.js:93:5)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:8240) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 3)
(node:8240) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
```

Here is the code used:
```
GitHub = require("github-api");

github = new GitHub(username, password);
repo = github.getRepo(user, repo) // actual values are "kyllta", "urlshortener"

repo.writeFile(
"main",
"test_file.txt",
"test contents",
"test commit"
);
```

Any attempts to use in HTML code (Chrome) result in this error:
```
xhr.js:177 PUT https://api.github.com/repos/kyllta/urlshortener/contents/saved_urls.json 404
  | (anonymous) | @ | xhr.js:177
-- | -- | -- | --
  | t.exports | @ | xhr.js:13
  | t.exports | @ | dispatchRequest.js:50
  | Promise.then (async) |   |  
  | n.request | @ | Axios.js:61
  | (anonymous) | @ | bind.js:9
  | value | @ | Requestable.js:183
  | (anonymous) | @ | Repository.js:787
  | Promise.then (async) |   |  
  | value | @ | Repository.js:785
  | submit | @ | (index):54
  | async function (async) |   |  
  | submit | @ | (index):32
  | onclick | @ | (index):73

```
My suspicion is incorrect headers, but I don't know much.
#577 and #520 are similar, but have different errors, and so don't seem to apply.

Start with Repository.writeFile and the Requestable.js handler around the reported line, then compare the browser request in Repository.js with the supplied arguments and GitHub API response. Verify the fix by reproducing the writeFile call for the kyllta/urlshortener repository without an undefined status or 404 response.

索引モデルが issue の本文から書いたものです。