default_wt should be JSON not eval'd Ruby.

For both performance and security reasons, it's much better to default to using JSON rather than using Ruby code and invoking Ruby's parse and interpreter to eval responses from Solr.
Fix JSON parsing to handle blank response bodies (for testing.)
Merge branch 'change-default-wt-to-json' of github.com:jcoleman/rsolr into orange-function-customizations