Use var_export for scalar default values

[malarzm]

This PR will be good enough to cut a 3.4.2 release. It will:

• fix ProxyGenerator turns double quoted default values into single quoted default values #987 (also changes will need to stay until ReflectionParameter::__toString mangles special characters in strings php/php-src#9622 is resolved)
• stop breaking existing code like mentioned in Wrong proxy class generation with default values as constants referncing self #988 and give us more time to deal with current approach's shenanigans

It is worth mentioning that we will NOT support correctly double-quoted strings in complex default values until a fix in PHP is present (or a workaround is found). Given people never really had a chance to start using such values, I believe this is OK.

@ampaze @Lustmored may I ask you to test your apps with this PR instead of released 3.4.1?

[ampaze]

@malarzm PR works for me.

[Lustmored]

@malarzm it also works for me. Good job finding the fix so quickly, thank you!