Extend DefaultDispatcherServletCondition to check for a registration

...bean with no explicit @Bean DispatcherServlet. We still have to check
by bean name (slightly unfortunate, but we need to avoid instantiating
too early) so there's now another magic
bean name for the registration bean ("dispatcherServletRegistration")
that the user has to replace if he wants the registration without
defining a servlet @Bean

Fixes gh-482

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/DispatcherServletAutoConfigurationTests.java

@@ -54,6 +54,21 @@ public class DispatcherServletAutoConfigurationTests {
 		assertEquals("[/]", registration.getUrlMappings().toString());
 	}
 
+	@Test
+	public void registrationOverride() throws Exception {
+		this.context = new AnnotationConfigWebApplicationContext();
+		this.context.register(CustomDispatcherRegistration.class,
+				ServerPropertiesAutoConfiguration.class,
+				DispatcherServletAutoConfiguration.class);
+		this.context.setServletContext(new MockServletContext());
+		this.context.refresh();
+		ServletRegistrationBean registration = this.context
+				.getBean(ServletRegistrationBean.class);
+		assertEquals("[/foo]", registration.getUrlMappings().toString());
+		assertEquals("customDispatcher", registration.getServletName());
+		assertEquals(0, this.context.getBeanNamesForType(DispatcherServlet.class).length);
+	}
+
 	@Test
 	public void servletPath() throws Exception {
 		this.context = new AnnotationConfigWebApplicationContext();
@@ -95,4 +110,15 @@ public class DispatcherServletAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	protected static class CustomDispatcherRegistration {
+		@Bean
+		public ServletRegistrationBean dispatcherServletRegistration() {
+			ServletRegistrationBean registration = new ServletRegistrationBean(
+					new DispatcherServlet(), "/foo");
+			registration.setName("customDispatcher");
+			return registration;
+		}
+	}
+
 }