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-test/src/main/java/org/springframework/mock/http/client/MockClientHttpRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -66,6 +66,11 @@ public class MockClientHttpRequest extends MockHttpOutputMessage implements Clie
 		return this.httpMethod;
 	}
 
+	@Override
+	public String getMethodValue() {
+		return this.httpMethod.name();
+	}
+
 	public void setURI(URI uri) {
 		this.uri = uri;
 	}

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

@@ -83,6 +83,11 @@ public class MockServerHttpRequest extends AbstractServerHttpRequest {
 		return this.httpMethod;
 	}
 
+	@Override
+	public String getMethodValue() {
+		return this.httpMethod.name();
+	}
+
 	@Override
 	public String getContextPath() {
 		return this.contextPath;