Fix '#x0' string issue (issue #150)

[morgangiraud]

Fix issue #150

[subzey]

Please, note that #x0; is invalid in XML.
Entity must refer to valid Char and Char range doesn't include a null character.

[isaacs]

I'm perfectly happy to accept whatever you all decide is best, but every change needs to have a test.

[morgangiraud]

Yeah you're right, If i include a test with a broken example which got fixed by my PR, will it be ok ?

[isaacs]

Sure. You can copy the examples from the other files in the tests folder. Please use a minimal amount of xml; it's rather rough to deal with tests that are multi-gb of real-world xml docs ;)

[thiakil]

@subzey while it isn't valid once parsed, it is technically valid as a CharRef, just not the 'Well-formedness constraint'.

I believe we should still not barf on it in strict mode (whether we add a special case for not expanding/parsing it in strict mode or not) as it could be seen in the wild.

As for how to solve the issue, we are able to add a simple forward reference to the RegExp in question and the issue solves itself. See thiakil/sax-js@72ae06c (NB: next commits on that branch switches the test to strict mode and add more CharRefs to the test). Can make a pull request if wanted.