GH-1230 Ensure header names are case insensitive

Resolves #1230

Author: olegz

Diff for: spring-cloud-function-adapters/spring-cloud-function-serverless-web/src/main/java/org/springframework/cloud/function/serverless/web/ServerlessHttpServletRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023-2023 the original author or authors.
+ * Copyright 2023-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -727,7 +727,7 @@ public Enumeration<String> getHeaderNames() {
 	}
 
 	public void setHeader(String name, @Nullable String value) {
-		this.headers.set(name, value);
+		this.headers.add(name, value);
 	}
 
 	public void addHeader(String name, @Nullable String value) {

Diff for: spring-cloud-function-adapters/spring-cloud-function-serverless-web/src/test/java/org/springframework/cloud/function/serverless/web/RequestResponseTests.java

@@ -29,6 +29,7 @@
 import org.springframework.cloud.function.test.app.PetStoreSpringAppConfig;
 import org.springframework.cloud.function.test.app.PetStoreSpringAppConfig.AnotherFilter;
 import org.springframework.cloud.function.test.app.PetStoreSpringAppConfig.SimpleFilter;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatus;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -56,6 +57,18 @@ public void before() {
 	public void after() {
 		this.mvc.stop();
 	}
+	
+	@Test
+	public void validateCaseInsensitiveHeaders() throws Exception {
+		ServerlessHttpServletRequest request = new ServerlessHttpServletRequest(null, "GET", "/index");
+		request.setHeader("User-Agent", "iOS");
+		request.setHeader("uSer-Agent", "FOO");
+		request.setContentType("application/json");
+		request.setHeader("CoNteNt-tYpe", "text/plain");
+		
+		assertThat(request.getHeader("content-TYPE")).isEqualTo("application/json");
+		assertThat(request.getHeader("user-agenT")).isEqualTo("iOS");
+	}
 
 	@Test
 	public void validateFreemarker() throws Exception {