fix OAuth client credentials content type override (#957)

- Bugfix: OAuth client credentials token request now sends correct `application/x-www-form-urlencoded` content type
- Improvement: HTTP client Content-Type in additional headers now overrides the payload content type

3 files changed, 16 insertions, 2 deletions

diff --git a/src/zenhttp/clients/httpclientcurlhelpers.h b/src/zenhttp/clients/httpclientcurlhelpers.h
index 0605a30f6..cb5f5d9a9 100644
--- a/src/zenhttp/clients/httpclientcurlhelpers.h
+++ b/src/zenhttp/clients/httpclientcurlhelpers.h
@@ -255,8 +255,13 @@ BuildHeaderList(const HttpClient::KeyValueMap&							AdditionalHeader,
 		Headers = curl_slist_append(Headers, AuthHeader.c_str());
 	}
 
+	bool HasContentTypeOverride = AdditionalHeader->contains("Content-Type");
 	for (const auto& [Key, Value] : ExtraHeaders)
 	{
+		if (HasContentTypeOverride && Key == "Content-Type")
+		{
+			continue;
+		}
 		ExtendableStringBuilder<128> HeaderLine;
 		HeaderLine << Key << ": " << Value;
 		Headers = curl_slist_append(Headers, HeaderLine.c_str());
diff --git a/src/zenhttp/httpclient_test.cpp b/src/zenhttp/httpclient_test.cpp
index 67cbaea9b..deaeca2a8 100644
--- a/src/zenhttp/httpclient_test.cpp
+++ b/src/zenhttp/httpclient_test.cpp
@@ -414,6 +414,17 @@ TEST_CASE("httpclient.post")
 		CHECK_EQ(Resp.AsText(), "{\"key\":\"value\"}");
 	}
 
+	SUBCASE("POST with content type override via additional header")
+	{
+		const char* Payload = "test payload";
+		IoBuffer	Buf(IoBuffer::Clone, Payload, strlen(Payload));
+
+		HttpClient::Response Resp = Client.Post("/api/test/echo", Buf, ZenContentType::kJSON, {{"Content-Type", "text/plain"}});
+		CHECK(Resp.IsSuccess());
+		CHECK_EQ(Resp.AsText(), "test payload");
+		CHECK_EQ(Resp.ResponsePayload.GetContentType(), ZenContentType::kText);
+	}
+
 	SUBCASE("POST with CbObject payload round-trip")
 	{
 		CbObjectWriter Writer;
diff --git a/src/zenhttp/httpclientauth.cpp b/src/zenhttp/httpclientauth.cpp
index 0432e50ef..26a7298b3 100644
--- a/src/zenhttp/httpclientauth.cpp
+++ b/src/zenhttp/httpclientauth.cpp
@@ -50,8 +50,6 @@ namespace zen { namespace httpclientauth {
 
 			IoBuffer Payload{IoBuffer::Wrap, Body.data(), Body.size()};
 
-			// TODO: ensure this gets the right Content-Type passed along
-
 			HttpClient::Response Response = Http.Post("", Payload, {{"Content-Type", "application/x-www-form-urlencoded"}});
 
 			if (!Response || Response.StatusCode != HttpResponseCode::OK)