Pass Solr admin request parameters as parameters, not POST data.

The examples all show parameters being passed in GET parameters, and the
server is not expecting a request body with a GET request anyway, so
previously the parameters would be ignored.
Fix typo in the SolrCoreAdmin.create() config parameter.

The default value should be the standard XML config file name, not a typo!
We can only create Cores with a CREATE request, not a STATUS.