Adding specs for progress proc invocation

This adds in specs for the progress proc getting called by open-uri when
passed on the Shoes download object.

In addition to the basic test, it also checks for the case where the
content length is nil, which happens if the HTTP headers don't include
the content length.
Whitespace and indentation
Merge pull request #1 from jasonrclark/download_specs

Looks great, thanks!