refactor: use Object.hasOwnProperty

[curbengh]

skipduplicate key is only checked for its existence, its value is actually not relevant to its purpose, even when the value is undefined.

If the key is there, its value is never undefined though, but rather boolean, string or number.

$ hexo migrate rss path.xml --skipduplicate => args: { _: ['path.xml'], skipduplicate: true }
$ hexo migrate rss path.xml --skipduplicate undefined => args: { _: ['path.xml'], skipduplicate: 'undefined' }
$ hexo migrate rss path.xml --skipduplicate 123 => args: { _: ['path.xml'], skipduplicate: 123 }

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Object.prototype.hasOwnProperty is indeed slower than typeof, but it only runs once so it shouldn't cause noticeable latency. Thus, in this use case, better accuracy can be achieved without performance impact.

[coveralls]

Coverage remained the same at 95.833% when pulling 3cfcb0d on curbengh:hasownprop into f53388d on hexojs:master.