handle parameter marshaling & change body marshaling

[petri]

YAML does a lot of basic type (de)marshaling for us. However this is not enough for complex data structures that need to be marshaled as JSON or XML.

The marshaling should take part after parametrization (of body and/or params) and added as an optional included step in the parametrization.

HTTP body marshaling indication should be changed to follow the same syntax as well.

Suggested syntax using type and valuekeys:

params:
  myparamname:
    type: JSON
    value: [{"a": 1}]

The implementation should allow for specifying default encodings for both body and params separately, thus allowing for omitting the extra type and value structure. In such a case marshaling nevertheless changes the body into the canonical form (having type and value)