Conditionally htmlEscape chars based on encoding

This commit adds new htmlEscape methods that take the character encoding
as a parameter. According to specs and recommendations, the list of
chars to be html escaped depends on the encoding used in the response.
If the current char encoding supports chars natively, we shouldn't
escape those; of course, reserved chars (<,>,',",&) should always be
escaped.

See: http://www.w3.org/TR/html4/sgml/entities.html#h-24.3
See: spring-projects/spring-framework#385 by @candrews

Issue: SPR-9293