Allow +nil+ for kramdown math_engine and syntax_highlighter options

[mahrud]

This allows users to explicitly turn off math engine and syntax highlighter of kramdown, following #545.
I also applied the changes requested by @parkr to prevent non-nil values.

[parkr]

This is great! I'd love some test coverage for this:

1. Resulting configuration has nil value when set to nil
2. Resulting configuration has mathjax value when unset
3. Resulting configuration has mathjax value when set to unsupportedjax

Same set of tests for syntax_highlighter. This should be tested in spec/github-pages/configuration_spec.rb in the context "#effective_config" block, please!

[mahrud]

Thanks for the suggestion! Turns out just accepting +nil+ wouldn't work, because it is treated the same as not having specified anything, so now instead I check for "nil". Incidentally, kramdown is okay with getting "nil" and treats it like you'd expect, but just to make the test more standard I change "nil" to +nil+ after the merge with defaults has happened.

[mahrud]

@parkr is there anything else I could add?
The only alternative to using string "nil" to mean no mathjax was using something like false or "nomath" or some other constant, but "nil" seemed like the most reasonable one because kramdown gives an error if it can't find the math engine unless it is the string "nil".

[parkr]

I'd really much prefer that the user specifies nil and we explicitly check the user's config for this instead of the pre-merged values. I know we load things slightly different in GitHub Pages's code, but maybe we could go that route in both places to respect the true meaning of YAML.

[chrisyeh96]

Hi @parkr, are there updates on this pull request? I would love to see this merged in order to set math_engine: nil. Thanks!

[mahrud]

@parkr I didn't understand your last comment at the time and forgot about this. I just rebased the PR to fix the conflicts. I don't think the test failing has anything to do with my changes though.

The problem, as I see it, is that giving nil for math_engine is treated the same as giving no value, which is interpreted as using the default, i.e. mathjax, so changing this behavior would break people's website. That's why checking for the string "nil" as an alternative seemed reasonable. Would checking for "" (empty string) be better?