Add `seeSourceEquals()`

When asserting "small" pages, it sometimes can be handy to assert that the page source is *equal* to an expected string. REST module has a nice method for that: [`seeResponseEquals()`](https://github.com/Codeception/module-rest/blob/master/src/Codeception/Module/REST.php#L1258).
I'm suggesting:
* Move this code to `lib-innerbrowser` so all the other modules can use it too.
* Rename it to `seeSourceEquals()` (or `seePageSourceEquals()` - depending on the outcome of https://github.com/Codeception/module-webdriver/issues/19)
* Add `seeResponseEquals()` as an alias in REST module (for BC and semantics)

I suggested this for WebDriver as well: https://github.com/Codeception/module-webdriver/issues/20