Fix frozen string literal issue

To prepare for ruby 3.4.0 and making strings frozen by default, we add `+` operator to make the string mutable

Author: chaadow

lib/ruby_http_client.rb

@@ -184,8 +184,8 @@ def build_args(args)
     #   - The final url string
     #
     def build_url(query_params: nil)
-      url = [add_version(''), *@url_path].join('/')
-      url = build_query_params(url, query_params) if query_params
+      url = [add_version(+''), *@url_path].join('/')
+      url = build_query_params(+url, query_params) if query_params
       URI.parse("#{@host}#{url}")
     end