assert_dom strict parameter

[jyeharry]

• Add macos to supported platforms in Gemfile.lock
• Add failing test for assert_dom collapsing whitespace
• Add strict parameter to assert_dom

Fixes #121

Adds a :strict key to the equality test hash for assert_dom, so that we can specify whether or not to ignore excess whitespace just as the browser does.

I prefer the solution in #123. This was created since the issue I created specifically mentioned creating a :strict parameter.

[flavorjones]

OK, main is green, so please rebase when you have a chance.

[jyeharry]

@flavorjones Rebased and tests and ci is passing