fix($compile): handle boolean attributes in @ bindings

[jannic]

Commit db5e0ff handles initial values of boolean attributes. The same
change needs to be applied inside the attrs.$observe() call.

(I'm not familiar with the angular.js codebase, so this may not be the right way to fix the issue. What I verified is that this fixes a regression with some production code which worked with angular.js 1.2.28 and 1.3.20 but failed with 1.4.7.
So feel free to replace this PR with some better fix, as you like.)

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[gkalpak]

Good catch 👍 (This has been on my TODO list for a while 😃)
There are still a couple of things to do, before we can merge this:

1. Sign the CLA.
2. Add one or more tests, that verifies the fix (and prevents future regressions).

_{Feel free to ping me when/if you make any changes, so I can take another look 😉}

[jannic]

@gkalpak just signed the CLA a few minutes ago. A test was already added back in February.

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[jannic]

Rebased to current master and manually resolved conflicts with 874c0fd