benchmark test error

[carllhw]

after execute go test -v -bench=., got this error message.

goos: darwin
goarch: amd64
pkg: github.com/go-xorm/core
BenchmarkOriQuery-8                  	    1000	   1266147 ns/op
BenchmarkStructQuery-8               	    1000	   1921657 ns/op
BenchmarkStruct2Query-8              	     500	   2829271 ns/op
panic: interface conversion: interface {} is []uint8, not *string

goroutine 2903 [running]:
github.com/go-xorm/core.BenchmarkSliceInterfaceQuery(0xc42055b980)
	/Users/carllhw/Projects/go/src/github.com/go-xorm/core/db_test.go:232 +0x7b2
testing.(*B).runN(0xc42055b980, 0x1)
	/usr/local/Cellar/go/1.10.2/libexec/src/testing/benchmark.go:141 +0xb2
testing.(*B).run1.func1(0xc42055b980)
	/usr/local/Cellar/go/1.10.2/libexec/src/testing/benchmark.go:214 +0x5a
created by testing.(*B).run1
	/usr/local/Cellar/go/1.10.2/libexec/src/testing/benchmark.go:207 +0x80
exit status 2
FAIL	github.com/go-xorm/core	5.919s