Fix bug in explain for function_score queries.

The explain output for function_score queries with score_mode=max or
score_mode=min was incorrect, returning instead the value of the last
function.  This change fixes this.

For example, for the query:

    {
      "explain": true,
      "query": {
        "function_score": {
          "query": {
            "match_all": {}
          },
          "score_mode": "max",
          "functions": [
            {
              "boost_factor": 1
            },
            {
              "boost_factor": 3
            },
            {
              "boost_factor": 2
            }
          ]
        }
      }
    }

Results are returned containing

    hits: [
      {
       _score: 3,
       ...
       _explanation: {
          value: 2
          ...
      }
    ]

Similarly, if "score_mode": "min" is used, the value in the explanation is still 2.

I would expect the value in _explanation object to be the same as the _score value, and with this patch applied it is.