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a table and running it through W3C validator returns the following error: The border attribute on the table element is obsolete.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.
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"
FEATURE - Added support for server-side max file size by type for uploads

Fixed test case so it correctly checks body contents as well

Updated docs with the change
Merge branch '3.1' of git://github.com/Turnerj/silverstripe-framework into uploadfield-max-upload-size-by-type