Core: Arrays like [42] should fail .isNumeric()

Fixes #14179
(cherry picked from commit 10efa1f5b44046aab6bcc8423322a41923faa290)
Ajax: .load() should trim its selector

Fixes #14773
(cherry picked from commit 3a68c114e3a30af86dc261c9d62ce2ae4144d420)

Conflicts:
	src/ajax/load.js
Event: Allow triggerHandler(beforeunload)

Fixes #14791
(cherry picked from commit 06adf7c95d93507908181995a9ea15fe2729595a)

Conflicts:
	src/event.js
CSS: Return values should be numbers

Fixes #14792
(cherry picked from commit f4b37d89820535d6c7503925aa8872645681a865)
Core: Do not run window.onready when ready

Fixes #14802
(cherry picked from commit 2df1aad6a1c9376c2a477eba26ee992113ed1c23)
Ajax: Use two-arg form of .slice() for old IE

Followup to a4b9bc57, #14773
Build: Fix AMD option for custom builds

Fixes #14859
Ajax, Effects: Disable Chrome 31 workarounds
Css: Revert 24e587929f62428e1959b10aace6dc4fd65ab397

The workaround to be able to change !important styles broke the browser
keeping the old CSS value if the new one was rejected. Patching it would
involve a significant perf hit (~33%) so the initial patch needs to be
reverted instead.

Tests by m_gol & gibson042.

(cherry-picked from 10e654218a65ec9a0ffc875137cea945389c69f1)

Fixes #14836
Closes gh-1532
Support: Simplify the box-sizing test

Don't require body in the support.boxSizing test.