fix(ngSanitize): ensure decodeEntities clears parts[0]

Due to weirdness in Safari 7, the browser seems unable to clear the first 
element of the array of regexp matches when not stepping through line by line.

I have no real explanation for this except that perhaps parts[0] is a 
read-only property now.

This work-around solves the issue, but there may be a better way.

Closes #5192