Add test for improve coverage

[naokiy]

Add test for coverage.

• post.create() with data.content
• post.create() with callback
• post.publish() with callback

[coveralls]

Coverage increased (+0.08%) to 94.73% when pulling 9d13393 on naokiy:coverage_post_js into 56bfc43 on hexojs:master.

[naokiy]

Test was failed at AppVeyor.
I think the reason is gulpjs/v8flags#17 .

[coveralls]

Coverage increased (+0.13%) to 94.78% when pulling 282cb77 on naokiy:coverage_post_js into 56bfc43 on hexojs:master.

[coveralls]

Coverage increased (+0.13%) to 94.78% when pulling 282cb77 on naokiy:coverage_post_js into 56bfc43 on hexojs:master.