Fix custom elements attribute application, improve key checking

- Fix custom elements attribute application to acknowledge that not all
  custom elements operate purely based on attributes. (Plus, those
  blasted things are verbose as heck when you're working with them in
  raw form. It's also not that uncommon for functionality to be exposed
  via property and *not* attribute.)
- Don't memoize the normalized value when we 1. only use it once in each
  branch, and 2. only use it for a few special cases.
- Centralize the "has property key" code, so it's easier to tune and
  read. I also inlined a couple functions while I was at it since they
  were small and only used once.
- Actually test for how attributes are applied to raw DOM elements vs
  when we choose to use keys. When I first developed the patch, it
  silently worked, when I should've been breaking things.

docs/change-log.md

@@ -28,6 +28,7 @@
 - hyperscript: when an attribute is defined on both the first and second argument (as a CSS selector and an `attrs` field, respectively), the latter takes precedence, except for `class` attributes that are still added together. [#2172](https://github.com/MithrilJS/mithril.js/issues/2172) ([#2174](https://github.com/MithrilJS/mithril.js/pull/2174))
 - stream: when a stream conditionally returns HALT, dependant stream will also end ([#2200](https://github.com/MithrilJS/mithril.js/pull/2200))
 - render: remove some redundancy within the component initialization code ([#2213](https://github.com/MithrilJS/mithril.js/pull/2213))
+- render: Align custom elements to work like normal elements, minus all the HTML-specific magic. ([#2221](https://github.com/MithrilJS/mithril.js/pull/2221))
 
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