Don't require 'text' field

Remove the default field setting from the select handler. User must
either supply a field or use the 'df' request parameter.

Update the ping handler to use _yz_id which is required to be present in
the schema.
Update bench driver to pass df param

Now that solrconfig.xml does not specify a default field the bench
driver must pass one.
Update RS migration test to set default field

The text extractors between RS and YZ use a different default field.
Merge pull request #313 from basho/bugfix/rz/dont-require-text-field

Don't require the 'text' field