Mappings: Fix Get field mapping api with pretty flag

Use  builder.field method instead of XContentHelper#writeRawField

Closes #6552
@rjernst  I see...
I have no idea for good assertion.
Do you have any other idea?

Output without pretty
```
{
  "index" : {
    "mappings" : {
      "type" : {
        "obj.subfield" : {
          "full_name" : "obj.subfield",
          "mapping":{"subfield":{"type":"string","index":"not_analyzed"}}
        },
        "field1" : {
          "full_name" : "field1",
          "mapping":{"field1":{"type":"string"}}
        }
      }
    }
  }
}
```

Output with pretty
```
{
  "index" : {
    "mappings" : {
      "type" : {
        "obj.subfield" : {
          "full_name" : "obj.subfield",
          "mapping" : {
            "subfield" : {
              "type" : "string",
              "index" : "not_analyzed"
            }
          }
        },
        "field1" : {
          "full_name" : "field1",
          "mapping" : {
            "field1" : {
              "type" : "string"
            }
          }
        }
      }
    }
  }
}
```