Document syntax is modified to XWiki Syntax 1.0 when editing if the syntax is not in the list of configured syntaxes

To reproduce:

• Get a clean XE jetty/hsqldb distribution
• Remove xwiki/2.1 from the configured syntax list
• Restart your wiki
• Go to Dashboard/WebHome (it's in 2.1 syntax)
• Edit and Save (no need to make any change)
• Try to edit un wysiwyg mode for ex and you'll get the old wysiwyg editor since the doc syntax has been changed to 1.0