Prevent replacement pattern matches in assets.

[joevennix]

Reproduce: Try to minimize a document that has a <script> tag that points to the following code:

var $=1;if(2>$&&true)return;

You'll get an error, because of this line. The $& is treated as a reference to the match in the regex. The fix is to pass a function that returns the string, which does not have pattern matches interpreted.

[jrit]

Thanks @joevennix - change looks good, I'm going to merge it in and publish a release in the near future.