I add update `Set` method and unit test.

e.g: 
```C#
var sb = new SqlBuilder()
       .Set("vip = @vip", new { vip })
       .Set("updatetime = @updatetime", new { updatetime })
       .Where("id = @id", new { id })
;
var template = sb.AddTemplate("update #Users /**set**/ /**where**/");
connection.Execute(template.RawSql, template.Parameters);
```

RawSql will generate :　
```
update #Users Set vip = @vip , updatetime = @updatetime\n WHERE id = @id\n
```

using Dapper Execute in sqlserver it'll reqeust by below sql command
```
exec sp_executesql N'update #Users Set vip = @vip , updatetime = @updatetime
 WHERE id = @id
',N'@vip bit,@updatetime datetime,@id int',@vip=1,@updatetime='2020-01-01 00:00:00',@id=1
```
Co-authored-by: Nick Craver <nrcraver@gmail.com>
Co-authored-by: Nick Craver <craver@stackoverflow.com>