feat: Normalize HTTP headers to lower case

HTTP headers are case-insensitive by spec. Different HTTP libraries
treat them differently — some preserve case, some change case. This
can cause unnecessary differences in AppMap between library versions.

This change normalizes all HTTP headers to lower case for consistency.

Author: dividedmind

agent/src/main/java/com/appland/appmap/reflect/HttpHeaders.java

@@ -32,13 +32,14 @@ default Enumeration<String> getHeaderNames() {
 
   /**
    * Non-standard utility method. Retrieves all headers.
+   * Header names are normalized to lowercase since HTTP headers are case-insensitive.
    */
   default Map<String, String> getHeaders() {
     HashMap<String, String> headers = new HashMap<>();
 
     for (Enumeration<String> e = getHeaderNames(); e.hasMoreElements();) {
       String headerName = (String) e.nextElement();
-      headers.put(headerName, this.getHeader(headerName));
+      headers.put(headerName.toLowerCase(), this.getHeader(headerName));
     }
 
     return headers;

agent/test/petclinic/petclinic.bats

@@ -211,7 +211,8 @@ setup() {
   assert_success
   stop_recording
 
-  assert_json_eq '.events[] | .http_server_response | .headers["Content-Type"]' "text/html;charset=UTF-8"
+  # HTTP headers are case-insensitive and are normalized to lowercase
+  assert_json_eq '.events[] | .http_server_response | .headers["content-type"]' "text/html;charset=UTF-8"
 }
 
 @test "recordings capture elapsed time" {