#157 change to Request$new() call within RequestHandler$initialize

uses webmockr::pluck_body which returns NULL if the body is empty
if pluck_body gives NULL, change it to an empty string "" to match what is on disk in cassettes

R/request_handler.R

@@ -90,7 +90,7 @@ RequestHandler <- R6::R6Class(
       self$request_original <- request
       self$request <- {
         Request$new(request$method, request$url$url %||% request$url,
-          webmockr::pluck_body(request), request$headers,
+          webmockr::pluck_body(request) %||% "", request$headers,
           disk = !is.null(request$output$path))
       }
       self$cassette <- tryCatch(current_cassette(), error = function(e) e)