added checks to getters on lists so that they never return a null result, made QueryParameter and Header extend Parameter
fixed formatter
changed Method to String, created an abstract class Method to list the HTTP methods
fixed error in Introspector
put produces/consumes at operation level (https://github.com/restlet/apispark/issues/1054)
updated introspector to complete attributes produces/consumes (https://github.com/restlet/apispark/issues/1054)
applied changes of branch rwadef