title: defining custom mapping in searchkick does not work
body: <p>I have a Campaign model and this is how my custom mapping looks like:</p>
  
  <pre><code>searchkick mappings: { 
   campaign: {
     properties: {
       title: {type: "string", analyzer: "snowball"},
       deal_title: {type: "string", analyzer: "snowball"},
       description: {type: "string", analyzer: "snowball"}
     }                                         
   }                                           
  }
  </code></pre>
  
  <p>when I reindex campaign documents, and do</p>
  
  <pre><code>c = Campaign.search "this is"
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>Searchkick::InvalidQueryError: [400] {  
     "error":{  
        "root_cause":[  
           {  
              "type":"query_parsing_exception",
              "reason":"[match] analyzer [searchkick_search] not found",
              "index":"campaigns_development_20160205130806185",
              "line":1,
              "col":96
           }
        ],
        "type":"search_phase_execution_exception",
        "reason":"all shards failed",
        "phase":"query",
        "grouped":true,
        "failed_shards":[  
           {  
              "shard":0,
              "index":"campaigns_development_20160205130806185",
              "node":"kiTXKzIEQjuI9UNDzXZ1DA",
              "reason":{  
                 "type":"query_parsing_exception",
                 "reason":"[match] analyzer [searchkick_search] not found",
                 "index":"campaigns_development_20160205130806185",
                 "line":1,
                 "col":96
              }
           }
        ]
     },
     "status":400
  }
  </code></pre>
  
  <p>Also when I add merge_mappings: true to the existing mapping,</p>
  
  <pre><code>searchkick merge_mappings: true, mappings: { 
   campaign: {
     properties: {
       title: {type: "string", analyzer: "snowball"},
       deal_title: {type: "string", analyzer: "snowball"},
       description: {type: "string", analyzer: "snowball"}
     }                                         
   }                                           
  }
  </code></pre>
  
  <p>and </p>
  
  <pre><code>search c = Campaign.search "this is"
  </code></pre>
  
  <p>, I get results with campaigns having words "this is" in either of title/deal_title/description.</p>
  
  <p>But since I am using snowball analyzer, these common stop words should have never been indexed and thus return 0 result.</p>
  
  <p>Also in the second scenario with mapping changed, it gives no error. So I think that searchkick does not identify snowball analyzer and since I did merge_mappings: true, it must have used default analyzer for indexing. And thats why it does not give any error.</p>
  
  <p>So the question really is: Is there something wrong in first mapping that I showed or does searchkick does not identify snowball(or any analyzer for that matter, tried the same thing with stop analyzer) analyzer?</p>