Update lib/appsignal/hooks/http.rb
Update lib/appsignal/hooks/http.rb

Verify the existence of the `HTTP::Client` to ensure the `http.rb` gem is also used.

Co-authored-by: Tom de Bruijn <96835dd8bfa718bd6447ccc87af89ae1675daeca@tomdebruijn.com>
appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby