Flatten body so hackney can see it as empty

Author: pvsr

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unpublished
+
+- Fixed unnecessary `Content-Type` and `Content-Length` headers being added to
+  requests with empty bodies.
+
 ## v1.3.0 - 2024-12-07
 
 - Updated for `gleam_stdlib` v0.45.0.

src/gleam_hackney_ffi.erl

@@ -4,7 +4,7 @@
 
 send(Method, Url, Headers, Body) ->
     Options = [{with_body, true}],
-    case hackney:request(Method, Url, Headers, Body, Options) of
+    case hackney:request(Method, Url, Headers, iolist_to_binary(Body), Options) of
         {ok, Status, ResponseHeaders, ResponseBody} -> 
             {ok, {response, Status, ResponseHeaders, ResponseBody}};