Get field mapping api doesn't honor pretty flag

[javanna]

The problem is due to the use of XContentHelper#writeRawField in GetFieldMappingsResponse which is great when dealing with the _source field as we never want to prettify it, but in this case the output should be prettified if requested.

Steps to reproduce:

curl -XPUT localhost:9200/index/type/1 -d '{
   "foo":"bar"
 }
 '

curl localhost:9200/_mapping/field/foo?pretty

{
  "index" : {
    "mappings" : {
      "type" : {
        "foo" : {
          "full_name" : "foo",
          "mapping":{"foo":{"type":"string"}}
        }
      }
    }
  }
}