Fix misleading test case

The index value for "index the value exactly as specified" is `not_analyzed ` : http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/mapping-intro.html#_literal_index_literal

However, in indices/putMapping_test.go it's using: `un_analyzed ` which is invalid.

It's a bit misleading for people trying to use PutMapping by reading the test case like me. :p