add separate method to write the body of objectmapper mapping

this is necessary for our ArrayMapper which explicitly encodes array fields of type array,
as it creates mapping like this:

array_field: {
  type: array,
  inner: {
    type: object,
    dynamic: true,
    properties: {
      ...
    }
  }

and for this only the body without field_name: { ... } is needed.
Merge pull request #26 from crate/m/object_doxcontent

add separate method to write the body of objectmapper mapping