Fix case sensitivity issue in ServletRequestHeadersAdapter

The RequestHeaderOverrideWrapper did not deduplicate in keySet()
across underlying headers and overrides.

A similar change in size() even if it was working correctly,
to align with keySet and make it more efficient.

Closes gh-36418

spring-web/src/test/java/org/springframework/http/server/ServletRequestHeadersAdapterTests.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2002-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.http.server;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.util.MultiValueMap;
+import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Unit tests for {@link ServletRequestHeadersAdapter}.
+ */
+public class ServletRequestHeadersAdapterTests {
+
+	private final MockHttpServletRequest request = new MockHttpServletRequest();
+
+	private final MultiValueMap<String, String> headersAdapter = ServletRequestHeadersAdapter.create(request);
+
+
+	@Test // gh-36418
+	void caseSensitiveOverride() {
+		request.addHeader("foo", "value");
+		headersAdapter.set("Foo", "override value");
+
+		assertThat(headersAdapter.size()).isEqualTo(1);
+		assertThat(headersAdapter.keySet()).containsExactly("Foo");
+		assertThat(headersAdapter.getFirst("foo")).isEqualTo("override value");
+		assertThat(headersAdapter.get("foo")).containsExactly("override value");
+	}
+
+}