Add test that shows default_escape accepts lower-case values.

The docs and codes show that default_escape accepts only upper case
values, but this is in conflict that is made to the reference to
accepting the that values that "escape" accepts. escapes documents
accepting lower-case values instead.
Add test for default_escape => 'none'

This parallels what is accepted by 'escape'.

As we are transitioning to escape by default, we have one method that
sets a default of escape = none, and one that sets a default of escape = 'html'.

In our case, we need to override a a setting of default_escape='html'
with a default_escape='none', but this is currently not possible.
Update default_escape implementation to follow the docs.

It refers to accepting the options that 'escape' does, but the escape
docs show lowercase values, while default_escape checks for uppercase
values. (Either should do).

Also, 'escape' documents 'none' as a valid option, but 'default_escape'
did not accept it.