Query DSL: Cache range filter on date field by default

A range filter on a date field with a numeric `from`/`to` value is **not** cached by default:

    DELETE /test

    PUT /test/t/1
    {
      "date": "2014-01-01"
    }

    GET /_validate/query?explain
    {
      "query": {
        "filtered": {
          "filter": {
            "range": {
              "date": {
                "from": 0
              }
            }
          }
        }
      }
    }

Returns:

    "explanation": "ConstantScore(no_cache(date:[0 TO *]))"

This patch fixes as well not caching `from`/`to` when using `now` value not rounded.
Previously, a query like:

    GET /_validate/query?explain
    {
      "query": {
        "filtered": {
          "filter": {
            "range": {
              "date": {
                "from": "now"
                "to": "now/d+1"
              }
            }
          }
        }
      }
    }

was cached.

Also, this patch does not cache anymore `now` even if the user asked for caching it.
As it won't be cached at all by definition.

Added as well tests for all possible combinations.

Closes #7114.