fix: unexpected 404

[cheng-kang]

Closes #664

Issue

prerenderEmbed is using the actual cached value when there is a hit. The value (an array) gets cleared somehow during processing https://github.com/docsifyjs/docsify/blob/master/src/core/render/index.js#L159. By the time the cached value is hit again, though the value is not undefined, the content is empty. Hence 404 shows up.

Solution

Using a copy of cached value solves the problem.

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