Fix Invalid Comment Edge Case

[isomarcte]

According to the XML specification XML comments can not end in --->. Prior to this commit creating a comment of the form Comment("invalid-") would yield no error though it should, since it is rendered as <!--invalid--->. This commit makes such String values yield an IllegalArgumentException.

A companion object was also added with two helper methods, validated and validatedOpt, which allow for a user of the library to check if a String is valid without having to catch the IllegalArgumentException.

[isomarcte]

I had to have the Comment companion object extend scala.runtime.AbstractFunction1 in order to not break binary compatibility.

I'm also fine changing that to any of the following,

• Filter the binary compatibility issue since it is unlikely anyone is depending on that.
• Bumping the version.
• Removing the companion object and the Either/Option based validation code. Though this is my least preferred option. As a user of this library, I'd personally prefer to have the validation code. That being said, I'm happy to defer to the maintainers' preference.

[lrytz]

Thank you for that patch!

I took a quick look around the codebase, and there are many other constructors that perform validation. I think adding the new validation methods only for Comment and leaving everything else makes the library less consistent. So my preference would be to only add the check in the constructor, but no new public API. Once it's there it has to stay for bin compat...

[isomarcte]

@lrytz I've updated the PR and removed the companion object.

[lrytz]

Thanks!