Fix misleading test case

Hi, I'm recreating the PR after rebasing to the most current master.

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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