Fix reversing URLs on Local Site with template caching turned on.
A while back on RBCommons, we hit a bug after enabling template caching

where URLs weren't reversing properly. Reversing would seem to work at

times, and at other times, the page would break due to a lookup failure.

In these cases, it was always failing to find a URL with local_site_name

in the kwargs.
This turns out to be due to a bug on our end, and not something due to

us overriding the url template tag.
When building the cached nodes and rendering the first time, all would

be well, but then we'd set url_node.kwargs to equal self.kwargs in our

custom template tag. Future renders would re-use the instances of our

tag, and therefore the kwargs and the url_node attributes. kwargs would

be populated with a 'local_site_name' key, and all would fail from there

on out.
We just need to copy our args/kwargs when setting url_node, and not

assign directly.

Tested locally with template caching and a local site. I was able to

reliably trigger the failures before this change. After, I couldn't

make it happen once.