Don't auto-configure MultipartConfigElement when using Commons FileUpload

Previously, when a user had declared a custom MultipartResolver bean
that is a CommonsMultipartResolver, part resolution would fail. The
failure was occurring as the servlet container was consuming the parts
before CommonsMultipartResolver had a chance to read them. This was
happening because a MultipartConfigElement was being auto-configured.

This commit updates the multipart auto-configuration so that a
MultipartConfigElement is not auto-configured when there is a
CommonsMultipartResolver bean in the context.

Closes gh-7735

spring-boot-parent/pom.xml

@@ -54,6 +54,11 @@
 				<artifactId>log4j</artifactId>
 				<version>1.2.17</version>
 			</dependency>
+			<dependency>
+				<groupId>commons-fileupload</groupId>
+				<artifactId>commons-fileupload</artifactId>
+				<version>1.3.3</version>
+			</dependency>
 			<dependency>
 				<groupId>com.google.guava</groupId>
 				<artifactId>guava</artifactId>