[fix #777] Allow xml:lang="" by replacing datatype.languagecode by datatype.languagecode | string "".

[murata2makoto]

This pull request is to address #777.

[tofi86]

Thanks, Makoto san, for providing this fix. Can you probably also add a Java-based unit test for an XHTML file containing this empty lang attribute?

However, if I should better add this, please don't hestitate to say so and I will add a test during review this weekend.

Thanks.

[murata2makoto]

I am afraid that I have not written a Java-based unit test before. Could you take care of it this time?

[tofi86]

I am afraid that I have not written a Java-based unit test before. Could you take care of it this time?

Sure, no problem! 👍

[tofi86]

Test results with 4.0.2 results in error as expected and reported in #777:

>java -Duser.language=en -Duser.country=US -jar epubcheck.jar --mode xhtml -v 3.0 "src/test/resources/30/single/xhtml/valid/issue777-empty-lang.xhtml"
ERROR(RSC-005): /src/test/resources/30/single/xhtml/valid/issue777-empty-lang.xhtml/src/test/resources/30/single/xhtml/valid/issue777-empty-lang.xhtml(8,26): Error while parsing file 'value of attribute "lang" is invalid; must be an RFC 3066 language identifier'.
ERROR(RSC-005): /src/test/resources/30/single/xhtml/valid/issue777-empty-lang.xhtml/src/test/resources/30/single/xhtml/valid/issue777-empty-lang.xhtml(8,26): Error while parsing file 'value of attribute "xml:lang" is invalid; must be an RFC 3066 language identifier'.

Check finished with errors

epubcheck completed

The version from this PR does not report an error anymore. Travis tests are running, will merge when finished...

[tofi86]

Merged. Thanks!