Make RequestMappingHandlerMapping @Primary

Update WebMvcAutoConfiguration so that the RequestMappingHandlerMapping
bean is @Primary. Prior to this commit a NoUniqueBeanDefinitionException
would be thrown then using the MvcUriComponentsBuilder.

Fixes gh-2237
Merge branch '1.1.x'

Conflicts:
	spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/EndpointWebMvcAutoConfigurationTests.java
	spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.java