Github-PR-44 by richardfullmer: [PostgresPlatform] Fixing change detection when a default is removed

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

:

Url: https://github.com/doctrine/dbal/pull/44

Message:

Change detection when the default value is removed from a field is presently broken and produces invalid SQL.

This patch checks to see if a new default value actually exists before adding the SET DEFAULT '';

Uses DROP DEFAULT when the new default value does not exist