fix($http): don't remove content-type header if data is set by request transform

Fixes #7910
docs(CHANGELOG.md): add changes for 1.3.0-beta.14 and 1.2.19
feat(ngAnimate): conditionally allow child animations to run in parallel with parent animations

By default ngAnimate prevents child animations from running when a parent is performing an animation.
However there are a cases when an application should allow all child animations to run without blocking
each other. By placing the `ng-animate-children` flag in the template, this effect can now be put to
use within the template.

Closes #7946