Fix a bug which was causing Solr to return a 404 when trying to load the completions.

Before: `http://localhost:8983/solr/collection1//terms?q=happen&etc…`
After: `http://localhost:8983/solr/collection1/terms?q=happen&etc…`

This was causing an error with Solr 4.4.0. I haven't tested that fix with Solr < 4.4 but I guess not having a double / in the URL cannot harm ;)