Document boost not applied to numeric fields with ES

gistfile1.sh

#######
# creation of an index foo
#######

curl -XPUT 'http://127.0.0.1:9200/foo/?pretty=true' -d '
{
  "index" : {
    "number_of_shards" : 1,
    "number_of_replicas" : 0
  }
}
'

#Response:
#{
#  "ok" : true,
#  "acknowledged" : true
#}

#######
# mapping configuration of type bar
#######

curl -XPOST 'http://127.0.0.1:9200/foo/bar/_mapping?pretty=true' -d '
{
  "bar" : {
    "_boost" : { "name" : "_boost" },
    "properties" : {
      "id" : { "type" : "integer" },
      "tag" : { "type" : "string" }
    }
  }
}
'

#Response:
#{
#  "ok" : true,
#  "acknowledged" : true
#}

#######
# index first document
#######
curl -XPOST 'http://127.0.0.1:9200/foo/bar?pretty=true'  -d '
{
   "_boost" : 1.0,
   "id" : 123,
   "tag" : "aaa"
}
'

#Response:
#{
#  "ok" : true,
#  "_index" : "foo",
#  "_type" : "bar",
#  "_id" : "3W91_rF2Ru-z1jrUn0lyyw",
#  "_version" : 1
#}

#######
# index second document
#######
curl -XPOST 'http://127.0.0.1:9200/foo/bar?pretty=true'  -d '
{
   "_boost" : 2.0,
   "id" : 124,
   "tag" : "bbb"
}
'

#Response:
#{
#  "ok" : true,
#  "_index" : "foo",
#  "_type" : "bar",
#  "_id" : "iWUwuYfMTZyMAglLSwIJuQ",
#  "_version" : 1
#}

#######
# search for documents on string fields
#######
curl -XGET 'http://127.0.0.1:9200/foo/bar/_search?pretty=true' -d '
{
   "fields" : [],
   "query" : {
      "bool" : {
         "should" : [
            {
               "term" : {
                  "tag" : {
                     "value" : "aaa"
                  }
               }
            },
            {
               "term" : {
                  "tag" : {
                     "value" : "bbb"
                  }
               }
            }
         ]
      }
   }
}
'

#Response:
#{
#  "took" : 9,
#  "timed_out" : false,
#  "_shards" : {
#    "total" : 5,
#    "successful" : 5,
#    "failed" : 0
#  },
#  "hits" : {
#    "total" : 2,
#    "max_score" : 0.09001608,
#    "hits" : [ {
#      "_index" : "foo",
#      "_type" : "bar",
#      "_id" : "iWUwuYfMTZyMAglLSwIJuQ",
#      "_score" : 0.09001608
#    }, {
#      "_index" : "foo",
#      "_type" : "bar",
#      "_id" : "3W91_rF2Ru-z1jrUn0lyyw",
#      "_score" : 0.04500804
#    } ]
#  }
#}


# Document boosting works for search on string fields

#######
# search for documents on integer fields
#######
curl -XGET 'http://127.0.0.1:9200/foo/bar/_search?pretty=true' -d '
{
   "fields" : [],
   "query" : {
      "bool" : {
         "should" : [
            {
               "term" : {
                  "id" : {
                     "value" : "123"
                  }
               }
            },
            {
               "term" : {
                  "id" : {
                     "value" : "124"
                  }
               }
            }
         ]
      }
   }
}
'

#Response:
#{
#  "took" : 2,
#  "timed_out" : false,
#  "_shards" : {
#    "total" : 5,
#    "successful" : 5,
#    "failed" : 0
#  },
#  "hits" : {
#    "total" : 2,
#    "max_score" : 0.5,
#    "hits" : [ {
#      "_index" : "foo",
#      "_type" : "bar",
#      "_id" : "3W91_rF2Ru-z1jrUn0lyyw",
#      "_score" : 0.5
#    }, {
#      "_index" : "foo",
#      "_type" : "bar",
#      "_id" : "iWUwuYfMTZyMAglLSwIJuQ",
#      "_score" : 0.5
#    } ]
#  }
#}

# Document boosting does not work for search on numeric fields