Fix attribute casing better

[Conduitry]

Properly handles when attributes aren't already lowercase.

For completeness, also normalizes attributes when they are being serialized as part of static HTML, although that isn't actually necessary, as the browser will correct the case anyway.

[Conduitry]

I have no idea what these build failures are about. It looks like travis and appveyor are agreeing on them, and they're all related to transition tests.

[Conduitry]

These tests are failing because CI is using npm to install the dependencies instead of Yarn, and so is getting the latest versions of them, rather than the ones in the Yarn lockfile. It looks like the update from jsdom 11.5.1 to 11.6.0 is the culprit. I would guess this is related to "Added a window.performance implementation, including the basics of the High Resolution Time specification: performance.now(), performance.timeOrigin, and performance.toJSON()."

edit: Changing the window.performance test hack from window.performance = { now: () => raf.time }; to window.performance.now = () => raf.time; fixes all but one of the tests under the latest version of jsdom, so I'd say this is related.

[Conduitry]

Test failure stuff separated into #1127 / #1128

[codecov-io]

Codecov Report

Merging #1126 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1126   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files         125      126    +1     
  Lines        4522     4522           
  Branches     1459     1459           
=======================================
  Hits         4137     4137           
  Misses        163      163           
  Partials      222      222

Impacted Files	Coverage Δ
src/generators/nodes/Element.ts	94.42% <100%> (ø)	⬆️
src/utils/fixAttributeCasing.ts	100% <100%> (ø)
src/generators/nodes/Attribute.ts	85.49% <100%> (-0.44%)	⬇️

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