fix(encode_url): skip encode non-urls

[curbengh]

url.parse(str).slashes is used to avoid treating mailto: as a transfer protocol (e.g. http, ftp).

const { parse } = require('url')
console.log(parse('mailto:abc@example.com'))
/*
Url {
  protocol: 'mailto:',
  slashes: null,
  auth: 'abc',
  host: 'example.com',
  port: null,
  hostname: 'example.com',
  hash: null,
  search: null,
  query: null,
  pathname: null,
  path: null,
  href: 'mail:abc@example.com'
}
*/

This avoids null from being appended undesirably.

---

• Expected behavior

encodeURL('mailto:abc@example.com')
// mailto:abc@example.com

• Actual behavior

encodeURL('mailto:abc@example.com')
// mailto:abc@example.comnull

Address hexojs/hexo#3710 (comment)

[coveralls]

Coverage increased (+0.02%) to 95.122% when pulling d56cd07 on curbengh:mailto into 486f4a0 on hexojs:master.

[coveralls]

Coverage increased (+0.02%) to 95.122% when pulling 9e3af48 on curbengh:mailto into 486f4a0 on hexojs:master.