fix serialiser

[kennedymwavu]

closes #107

1. pass ... from res$json() to the serialiser.
2. allow default serialiser to accept the ... from res$json() as either:

   • a single list named opts ie.

     res$json(..., opts = list(optionA = "some value", optionB = "another value"))

   • key=value pairs ie.

     res$json(..., optionA = "some value", optionB = "another value")

   • if both opts & key=value pairs are supplied, the key=value pairs should override any value with the same key/name in opts. this is in line with the docs of yyjsonr::write_json_str() which state this:

     Arguments:
     
            x: the object to be encoded
     
         opts: Named list of serialization options. Usually created by
               ‘opts_write_json()’
     
          ...: Other named options can be used to override any options in
               ‘opts’. The valid named options are identical to arguments to
               ‘opts_write_json()’
     

3. add more descriptions, examples, & tests for the serialiser.
4. fix a failing test for the Response class.

[JohnCoene]

Thanks!