[GH-194] Small test fixes for PostgreSQL

This issue is created automatically through a Github pull request on behalf of posulliv:

Url: https://github.com/doctrine/dbal/pull/194

Message:

When running the test suite against a clean PostgreSQL install on a 64-bit system I hit 2 issues:

• dropDatabase calls fail unless a database is manually created
• TypeConversionTest::testIdempotentDataConversion fails expecting bigint type to be converted to a string. PostgreSQL seems to detect 32 vs 64 bit systems and casts correctly for bigint types.

This small patch allows the test suite to run successfully for me against clean MySQL & PostgreSQL installs.