Replace invalid xml utf-8 chars in tag contents.

[trepmag]

I was getting issue to validate some feeds and finally found that not all utf-8 chars are valid for xml: http://www.phpwact.org/php/i18n/charsets#xml

So this commit insert the function found in the above link and use it when a string must be inserted into content tags.

[mibe]

Yeah I can confirm this bug. This code reproduces it:

$TestFeed->setTitle("Test\x18Test");

The problem is that the commit works for usual content in tags, but not for attributes. This will not validate:

$TestFeed->setAtomLink('http://pubsubhubbub.appspot.com', "hub\x18");

I think it would be better to implement the filtering of the illegal chars in the makeNode() method instead.

[mibe]

Oops, wrong button... 😣

[trepmag]

Mibe, not sure you've been notified on the above commit...

[mibe]

Yes, I didn't get a notify. Thanks for your work!