fix(cbor): copy headers when building request

Author: kuhe

.changeset/friendly-parents-compare.md

@@ -0,0 +1,5 @@
+---
+"@smithy/core": patch
+---
+
+copy input headers when building RPCv2 CBOR request

packages/core/src/submodules/cbor/parseCborBody.ts

@@ -100,7 +100,10 @@ export const buildHttpRpcRequest = async (
     port,
     method: "POST",
     path: basePath.endsWith("/") ? basePath.slice(0, -1) + path : basePath + path,
-    headers,
+    headers: {
+      // intentional copy.
+      ...headers,
+    },
   };
   if (resolvedHostname !== undefined) {
     contents.hostname = resolvedHostname;