pass on params via ... in response parse() method to iconv

[sckott]

so you could do e.g.

x <- HttpClient$new(some url)
res <- x$get()
res$parse(sub = "")

where users can replace non-convertible bytes with some string, and use other iconv params

comes via https://github.com/ropensci/rnoaa/issues/305 - we probably don't want to use sub for users, but rather let them pass on args to iconv