Unwrap if necessary for MultipartHttpServletRequest

Before this commit RequestPartServletServerHttpRequest simply did an
instanceof check for MultipartHttpServletRequest. That hasn't failed
because request wrapping typically happens in filters before the
DispatcherServlet calls the MultipartResolver.

With Spring MVC Test and the Spring Security integraiton however,
this order is reversed since there we prepare the multipart request
upfront, i.e. there is no actual parsing.

The commit unwraps the request if necessary.

Issue: SPR-13317

spring-web/src/test/java/org/springframework/web/multipart/support/RequestPartServletServerHttpRequestTests.java

@@ -19,6 +19,9 @@ package org.springframework.web.multipart.support;
 import java.net.URI;
 import java.nio.charset.Charset;
 
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletRequestWrapper;
+
 import org.junit.Test;
 
 import org.springframework.http.HttpHeaders;
@@ -86,6 +89,20 @@ public class RequestPartServletServerHttpRequestTests {
 		assertArrayEquals(bytes, result);
 	}
 
+	// SPR-13317
+
+	@Test
+	public void getBodyWithWrappedRequest() throws Exception {
+		byte[] bytes = "content".getBytes("UTF-8");
+		MultipartFile part = new MockMultipartFile("part", "", "application/json", bytes);
+		this.mockRequest.addFile(part);
+		HttpServletRequest wrapped = new HttpServletRequestWrapper(this.mockRequest);
+		ServerHttpRequest request = new RequestPartServletServerHttpRequest(wrapped, "part");
+
+		byte[] result = FileCopyUtils.copyToByteArray(request.getBody());
+		assertArrayEquals(bytes, result);
+	}
+
 	// SPR-13096
 
 	@Test