title: mongoDB search Text error
body: <p>there is an error when using searching index, i use v3.4.</p>
  
  <pre><code>&gt; db.test2.insert({string:"aa bb cc"});
  WriteResult({ "nInserted" : 1 })
  &gt; db.test2.insert({string:"aa 88 99"});
  WriteResult({ "nInserted" : 1 })
  &gt; db.test2.ensureIndex({'string':"text"});
  
  
  {
          "ok" : 0,
          "errmsg" : "Index: { v: 2, key: { _fts: \"text\", _ftsx: 1 }, name: \"string_text\", ns: \"test2.test2\", weights: { string: 1 }, default_language: \"english\", language_override: \"language\", textIndexVersion: 3 } already exists with different options: { v: 2, key: { _fts: \"text\", _ftsx: 1 }, name: \"post_text_text\", ns: \"test2.test2\", weights: { post_text: 1 }, default_language: \"english\", language_override: \"language\", textIndexVersion: 3 }",
          "code" : 85,
          "codeName" : "IndexOptionsConflict"
  }
  </code></pre>