Support capitalized Content-Type header (#32)

Normalize all header keys to lowercase

Author: christiangaetano

.changeset/polite-goats-taste.md

@@ -0,0 +1,5 @@
+---
+'@as-integrations/aws-lambda': patch
+---
+
+Downcase all header keys during normalization

src/index.ts

@@ -169,7 +169,7 @@ function normalizeHeaders(
 ): Map<string, string> {
   const headerMap = new Map<string, string>();
   for (const [key, value] of Object.entries(headers)) {
-    headerMap.set(key, value ?? '');
+    headerMap.set(key.toLocaleLowerCase(), value ?? '');
   }
   return headerMap;
 }