Lowercase attribute names when storing them

[mojavelinux]

Currently, Asciidoctor lowercases attribute names when resolving them. However, it does not also lowercase attribute names passed in as arguments to the processor. As a result, it's not possible for a document to resolve an attribute that contains upper case characters in this case.

For example, given the following invocation:

$ asciidoctor -a publishURL=http://example.com sample.adoc

the following attribute reference will resolve to nil:

{publishURL}

That's because the stored name, publishURL, does not match the lookup name, publishurl.

[mojavelinux]

This is a bug fix, so it should not affect documentation (given that it's implementing the behavior already described).