support scan assign slice cap

[demoManito]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

support scan assign slice cap，instead of the default cap of 20

User Case Description

for example:

users := make([]User, 0, 2)
db.Where(...).Find(&users).Error

The initial make cap is 2, instead of 20. And there is no need to reallocate.