Replace '\uFEFF' with '!' for setInnerHTML to avoid UglifyJS bug

#2247 (and mishoo/uglifyjs2#556)

This is obviously quite a serious bug and even if uglifyjs fixes it, old versions will still be around to haunt people who minify themselves.

The comment should explain everything. The char is immediately removed so it should never affect rendering.

Test plan: none (should test in IE8)

cc @zpao @spicyj