Fix: WYSIWYG now returns valid HTML in tables

The WYSIWYG by default creates invalid HTML5. Creating a table and running it through W3C validator returns the following error: The border attribute on the table element is obsolete. Use CSS instead.
BUG Config::merge_array_low_into_high() ignores falsey values

Specific case: LeftAndMain::$session_keepalive_ping = true cannot be
set to false in config.yml for some cases because the value is ignored
when merge_array_low_into_high() is processing the config arrays.
Merge pull request #3272 from halkyon/config_fix

BUG Config::merge_array_low_into_high() ignores falsey values
Merge pull request #2998 from torleif/patch-4

Fix: WYSIWYG now returns valid HTML in tables
Revert "BUG Config::merge_array_low_into_high() ignores falsey values"
Merge pull request #3275 from silverstripe/revert-3272-config_fix

Revert "BUG Config::merge_array_low_into_high() ignores falsey values"