fix($httpBackend): set headers with falsy values

This is a breaking change. To migrate to the new behavior,
delete or set headers to `undefined` to avoid having them sent.
To restore the old behavior, override `$httpBackendProvider`
with the old implementation.

Closes #2984
docs(ngRoute): add angularEvent param to $routeChangeStart event

Adds missing implied first argument, `angularEvent`, to match
the rest of the `$routeChange` event documentation.
fix($compile): allow interpolations for non-event handlers attrs

Fix wrong behaviour that didn't allow 'data-on' and 'on' element attributes
to be interpolated by $compile. The regex now accepts any string beginning
with 'on' and with at least one more English letter.
feat($compile): support tel: links in a[href]

Allow `tel:` links so that click-to-call works in mobile browsers
test($sce): make ie8 happy

Ref: https://github.com/angular/angular.js/pull/4221#/issuecomment-25515813

Closes #4221
style($qSpec): add semi-colons
style(rootScopeSpec): add semi-colons
fix($compile): ng-attr to support dash separated attribute names
docs($compile): improve explanation of Attributes.$observe

The current comment of Attributes.$observe doesn't state correctly the behavior when the attribute contains no interpolation. Specifically, it states that the observer function will never be invoked if the attribute contains no interpolation. However, the actual behavior in this case is that the observer will be invoked once during the next digest loop.
fix($scope): $evalAsync executes on the right scope

Executes $evalAsync at the scope that the call was made

Closes: #3548
test($sce): ie8 fix for entire file

Ref: https://github.com/angular/angular.js/pull/4221#/issuecomment-25515813

Closes #4221
fix($location): prevent infinite digest error in IE7

Refactored `replacedUrl` to store the new URL on both
`location.replace` and setting `location.href` directly to handle
delays in the actual location value change in IE.

Closes #2802
feat(ngMock.$timeout): remove flushNext method
fix(isArrayLike): correctly handle string primitives

Closes #3356
fix($resource): pass transformed value to both callbacks and promises

Closes #3817
fix($sanitize): sanitize DOCTYPE declarations correctly

HTML to be sanitized that contains a DOCTYPE declaration were causing
the HTML parser to throw an error.  Now the parser correctly removes
the declarations when sanitizing HTML.

Closes #3931
docs(select): clarify usage

The wording seemed confusing and these adjustments seem to capture the intent
with less turbulence.

Closes #4257
docs(tutorial): update examples to show best practices

Closes #4256, #4255, #4254, #4253, #4250, #4092
docs(tutorial): improve wording and consistency
docs($compile): fix param description being displayed as code block

Closes #4187