Fix behaviour of ReflectionClass::export/ReflectionObject::export to match behaviour of core classes

[asgrim]

Replaces #397 from @muglug - I've rebased onto latest master, and implemented the proposal.

Note: introduces a BC break, so definitely targeting 4.0.0 for this.

[Ocramius]

@asgrim should we close the 3.x milestones?

[asgrim]

@Ocramius sure, if we want to go from 3.5 to 4.0 👍 I'm fine with that

[Ocramius]

Right, from now on master is only dedicated to 4.0.0. @asgrim @kukulich if you have patches with BC Breaks, don't be scared to break it ;-)

[Ocramius]

Fun times! Merging this resulted in following (something in dependencies):

There was 1 error:
1) Roave\BetterReflectionTest\Reflection\Adapter\ReflectionMethodTest::testAdapterMethods with data set #8 ('getDocComment', null, false, array())
TypeError: Return value of Mock_ReflectionMethod_15775ca8::getDocComment() must be of the type string, boolean returned
C:\Users\travis\build\Roave\BetterReflection\src\Reflection\Adapter\ReflectionMethod.php:109
C:\Users\travis\build\Roave\BetterReflection\test\unit\Reflection\Adapter\ReflectionMethodTest.php:130