Don't set NULL if timestamp column is Primary Key

[zaneli]

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• Do only one thing
• No API-breaking changes
• New code/logic commented & tested

For significant changes like big bug fixes, new features, please open an issue to make an agreement on an implementation design/plan first before starting it.

What did this pull request do?

type Time struct {
	Time time.Time `gorm:"primary_key"`
}

db.CreateTable(&Time{})

• before

(/Users/zaneli/go/src/github.com/zaneli/gorm/main.go:737)
[2019-03-14 16:47:06]  Error 1171: All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, use UNIQUE instead

(/Users/zaneli/go/src/github.com/zaneli/gorm/scope.go:1034)
[2019-03-14 16:47:06]  [0.97ms]  CREATE TABLE `times` (`time` timestamp NULL , PRIMARY KEY (`time`))

• after

(/Users/zaneli/go/src/github.com/zaneli/gorm/scope.go:1034)
[2019-03-14 16:47:35]  [6.00ms]  CREATE TABLE `times` (`time` timestamp , PRIMARY KEY (`time`))

[zaneli]

maybe since MySQL 5.7.3

https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-3.html

Columns in a PRIMARY KEY must be NOT NULL, but if declared explicitly as NULL produced no error. Now an error occurs.

[zaneli]

I fixed conflict for merge.

@jinzhu @emirb
Please take another look.