Polish formatting
Fix concurrent gaugeLocks map access

Use putIfAbsent to ensure atomic creation of lock objects.

Fixes gh-1995
Make ErrorPageFilter public

Change the visibility of ErrorPageFilter to public to fix
IllegalAccessException errors on certain servlet containers.

Fixes gh-2026
Merge branch '1.1.x'

Conflicts:
	spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java