Fix compatibility with Jekyll >= 1.1.0

[IQAndreas]

Adds an additional argument added in a newer Jekyll version. Luckily,
since the value is optional, this plugin is still compatible with older
versions of Jekyll.

Fixes #13

[parkr]

You won't get any attributes if you pass in nil.

[parkr]

#15 should do it.

[IQAndreas]

You won't get any attributes if you pass in nil.

Actually, you will, since Jekyll's version of to_liquid looks like this:

(attrs || self.class::ATTRIBUTES_FOR_LIQUID)

[IQAndreas]

#15 should do it.

Actually, your code isn't compatible with previous versions of Jekyll (and for reference, Octopress still uses Jekyll 0.12.0).

I added a fix for reverse-compatibility with previous versions in my fork, but had forgotten to add those fixes to this pull request.

See what I mean by "not compatible with other versions"? Damn monkey-patching dynamic languages.

[parkr]

@IQAndreas That's totally fine, as this is a plugin for Jekyll. My fix happens to be for the latest version of Jekyll, as the only rational assumption for me to make is that this plugin is compatible with Jekyll v1.x, as it's been out for many months now. Octopress is another story and I didn't consider it here for the reason you stated.