fix: default value with typecast and tests

[MrChaos1993]

• [x] Do only one thing
• [x] Non breaking API changes
• [x] Tested

What did this pull request do?

Fixing incorrect default value in auto generated models.

User Case Description

The current version(master) ignore default value if it contains type cast.

also will fix following PR: https://github.com/go-gorm/postgres/pull/210 https://github.com/go-gorm/postgres/pull/201

related issues: https://github.com/go-gorm/gorm/issues/6543

related PR: https://github.com/go-gorm/postgres/pull/143

[jinzhu]

Sorry for the confliction due to merge PR #210, can you fix them, thank you.