Default encapsulator param in_arg_hash not compatible with solr 6

[kshefchek]

The default encapsulator for get_download_url() is set to an empty string. This is no longer a valid encapsulator in solr 6.

[kltm]

I don't have milestones here yet--I'd need to know how urgent or take a PR.

[kshefchek]

Since it can be fixed by simply passing in a value to the function it's not really urgent from our end of things.

[kltm]

Gorf. Okay. As with many of these things, give me a poke if it becomes an irritant. Ideally the API Just Works and is expanded and used more ubiquitously.