Add `seeSourceEquals()`

[ThomasLandauer]

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(). 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