Fix: generated IDs are converted to integer

[pepakriz]

When I try to use type bigint and @GeneratedValue together, generated value is converted to integer.

See PHP documentation http://php.net/manual/en/language.types.array.php

Strings containing valid integers will be cast to the integer type. E.g. the key "8" will actually be stored under 8. On the other hand "08" will not be cast, as it isn't a valid decimal integer.

[doctrinebot]

Hello,

thank you for creating this pull request. I have automatically opened an issue
on our Jira Bug Tracker for you. See the issue link:

http://www.doctrine-project.org/jira/browse/DDC-3634

We use Jira to track the state of pull requests and the versions they got
included in.

[Ocramius]

Requires a test case

[Ocramius]

@pepakriz I've manually merged this after adding DDC3634Test 🚢

[pepakriz]

👍 Thanks