title: elasticsearch kibana update document which has space using update by query
body: <p>I have a field to update which has space in it.</p>
  
  <pre><code>POST /index/type/_update_by_query
  {
    "query": {
        "match_phrase":{
          "field": "value"
        }
    },
    "script":{
      "lang": "painless",
      "inline": "ctx._source.Existing Field = New_Value"
    }
  }
  </code></pre>
  
  <p>But I get this error.</p>
  
  <pre><code>{
    "error": {
      "root_cause": [
        {
          "type": "script_exception",
          "reason": "compile error",
          "script_stack": [
            "ctx._source.Existing Field = New_Value",
            "                     ^---- HERE"
          ],
          "script": "ctx._source.Existing Field = New_Value",
          "lang": "painless"
        }
      ],
      "type": "script_exception",
      "reason": "compile error",
      "script_stack": [
        "ctx._source.Existing Field = New_Value",
        "                     ^---- HERE"
      ],
      "script": "ctx._source.Existing Field = New_Value",
      "lang": "painless",
      "caused_by": {
        "type": "illegal_argument_exception",
        "reason": "unexpected token ['Field'] was expecting one of [{&lt;EOF&gt;, ';'}]."
      }
    },
    "status": 500
  }
  </code></pre>
  
  <p>When I execute this query on a field which doesn't have space, it works fine.
  How do I handle cases where there is a space in the field name?</p>
  
  <p>ELK version = 5.4.3
  I have read in the documentation that using spaces in field names is not advised, but these fields are dynamically created from a certain server and there are like 1M data entries every day. Hence I want to do a update_by_query on all the matching entries.</p>