Fix for #3515, functional extension & tests

https://cakephp.lighthouseapp.com/projects/42648-cakephp/tickets/3515

I used (string) because it is faster than strval() and there are no real benefits in this case.
Also enabled assign(), append() and prepend() to take any values convertible to string.

Removed try/catch as discussed with ADmad.
Changed the three exception expecting tests to check for PHPUnit_Framework_Error now.
Fixed duplication of Model's afterFind callback in HABTM relation and content of  array passed to that callback [ticket 3970]
Problems with files :/
Fixed tabing formating
Fixed tabbing in some places
Some more space / tabs issues [Travis PHPCS=1 fail]
Added some tests for related models afterFind callback
Some tabbing issues
Added some tests to related models afterFind callback
I'm a git noob ;/
Added some test for related model afterFind callback
Added some test for related model afterFind callback
Deleted rows in ModelReadTest
Formatting issues
Formatting issues
Formatting issues
Formatting issues
Formatting issues
Formatting issues
Remove added fixtures and use existing. Updated one old test to add data from afterFind callback