Allow %URI.DefaultScheme to be null.

Fixes #103.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

library/HTMLPurifier/ConfigSchema/schema/URI.DefaultScheme.txt

@@ -1,5 +1,5 @@
 URI.DefaultScheme
-TYPE: string
+TYPE: string/null
 DEFAULT: 'http'
 --DESCRIPTION--
 
@@ -7,4 +7,9 @@ DEFAULT: 'http'
     Defines through what scheme the output will be served, in order to
     select the proper object validator when no scheme information is present.
 </p>
+
+<p>
+    Starting with HTML Purifier 4.9.0, the default scheme can be null, in
+    which case we reject all URIs which do not have explicit schemes.
+</p>
 --# vim: et sw=4 sts=4