Add HttpRequest.getMethodValue

This commit introduces a new method in HttpRequest:
`String getMethodValue`, which returns the HTTP method as a String.
Furthermore, HttpRequest.getMethod() has been given a default
implementation using this String value in combination with
`HttpMethod.resolve`.

Issue: SPR-15545

spring-web/src/test/java/org/springframework/http/client/InterceptingClientHttpRequestFactoryTests.java

@@ -267,6 +267,11 @@ public class InterceptingClientHttpRequestFactoryTests {
 			return method;
 		}
 
+		@Override
+		public String getMethodValue() {
+			return method.name();
+		}
+
 		public void setMethod(HttpMethod method) {
 			this.method = method;
 		}

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

@@ -31,7 +31,6 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.NettyDataBufferFactory;
 import org.springframework.http.HttpCookie;
 import org.springframework.http.HttpHeaders;
-import org.springframework.http.HttpMethod;
 import org.springframework.util.Assert;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
@@ -93,8 +92,8 @@ public class RxNettyServerHttpRequest extends AbstractServerHttpRequest {
 	}
 
 	@Override
-	public HttpMethod getMethod() {
-		return HttpMethod.valueOf(this.request.getHttpMethod().name());
+	public String getMethodValue() {
+		return this.request.getHttpMethod().name();
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/mock/http/server/reactive/test/MockServerHttpRequest.java

@@ -82,6 +82,11 @@ public class MockServerHttpRequest extends AbstractServerHttpRequest {
 		return this.httpMethod;
 	}
 
+	@Override
+	public String getMethodValue() {
+		return this.httpMethod.name();
+	}
+
 	@Override
 	public String getContextPath() {
 		return this.contextPath;