Fix exponential DOM issue in unwrap

[bfontaine]

Hi Mat,
This is the PR for the fix. I copy the explanation below (also in the commit message) for other readers, but it’s the same explanation that I sent you via email.
I didn’t include unit tests as you said you would write them; tell me if you want some.

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This fixes an issue in the current implementation where if sanitize is called on an HTML with deep nested unallowed elements it creates an exponential number of new elements and can eventually exhaust the available memory.

For example, if you have this HTML:

<x><y><z>blabla</z></y></x>

The code first hits <x> and inserts its .innerHTML after it:

<x><y><z>blabla</z></y></x>
<y><z>blabla</z></y>

Then it hits the inner <y>, and same thing:

<x><y><z>blabla</z></y>
  <z>blabla</z></x>
<y><z>blabla</z></y>

Then the inner <z>:

<x><y><z>blabla</z>
      blabla</y>
  <z>blabla</z></x>
<y><z>blabla</z></y>

Then the first <z>’s copy, etc.
At the end, there are 7 nodes in the document (2^3-1). If you raise the depth from 3 to 20, you get 2^20-1=1M nodes.

Note that there is also a performance issue as .innerHTML serializes the whole DOM and then insertAdjacentHTML unserializes it.

This PR fixes both the exponential issue and the serialization/unserialization.

[mat-sz]

Thank you for the research. I'll release this in the next patch version.

[mat-sz]

Available in version v1.0.8. Thank you again.