fix payload and add sample

sample/login.js

@@ -0,0 +1,11 @@
+const {Authentication, Transport} = require('../src');
+
+const transport = new Transport('http://localhost:3000');
+const auth = new Authentication(transport);
+auth.loginWithToken(process.env.GITHUB_ACCESS_TOKEN)
+  .then(() => {
+    console.log('login success');
+  })
+  .catch((error) => {
+    console.error(`login fails: ${error}`);
+  })

src/authentication.js

@@ -60,7 +60,7 @@ class Authentication {
     invariant(_.isString(token), 'token should be string');
     invariant(_.isString(service), 'service should be string');
     return this._transport
-      .post(`/auth/${service}/token`, {token}, {})
+      .post(`/auth/${service}/token`, {access_token: token}, {}) // eslint-disable-line camelcase
       .then((response) => {
         invariant(response.data.token, 'there should be token');
         debug(`Login response: ${JSON.stringify(response.data)}`);