Add support for sorting json keys

Add `jsonSortKeys` property to HttpMapperProperties.

Fixes gh-498

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -64,6 +64,7 @@ server.tomcat.max-threads = 0 # number of threads in protocol handler
 
 # SPRING MVC (HttpMapperProperties)
 http.mappers.json-pretty-print=false # pretty print JSON
+http.mappers.json-sort-keys=falose # sort keys
 spring.view.prefix= # MVC view prefix
 spring.view.suffix= # ... and suffix
 spring.resources.cache-period= # cache timeouts in headers sent to browser