title: illegal_argument_exception: Can only use phrase prefix queries on text fields - not on [_id] which is of type [_id]
body: <p>When I search using the following on ES 6.3.2, no problems:
  {"query": {"match_phrase_prefix" : {"_id":"ZX-bLmsBFOiwdS-Iwr24"}}}</p>
  
  <p>But when I do it on ES 7.0.1, it throws exception as follows:
  "type": 
  "illegal_argument_exception"</p>
  
  <p>"reason": 
  "Can only use phrase prefix queries on text fields - not on [_id] which is of type [_id]"</p>