fix(client): send content-type header for requests with an omitted optional body

Author: stainless-app[bot]

lib/lithic/internal/transport/base_client.rb

@@ -306,7 +306,10 @@ def initialize(
               Lithic::Internal::Util.deep_merge(*[req[:body], opts[:extra_body]].compact)
             end
 
-          headers.delete("content-type") if body.nil?
+          # Generated methods always pass `req[:body]` for operations that define a
+          # request body, so only elide the content-type header when the operation
+          # has no body at all, not when an optional body param was omitted.
+          headers.delete("content-type") if body.nil? && !req.key?(:body)
 
           url = Lithic::Internal::Util.join_parsed_uri(
             @base_url_components,