Modify XML encoder to hex-encode invalid UTF-8 sequences

There are still some holes, e.g. we leave surrogate pairs be
even though they are not a part of valid UTF-8, but this might
be for the better -- WTF-8 does support surrogate pairs inside
text.

Closes #1207

include/internal/catch_xmlwriter.h

@@ -56,7 +56,7 @@ namespace Catch {
 
         XmlWriter( std::ostream& os = Catch::cout() );
         ~XmlWriter();
-        
+
         XmlWriter( XmlWriter const& ) = delete;
         XmlWriter& operator=( XmlWriter const& ) = delete;