* fix race condition

This fixes a race condition I experience
```
WARNING: DATA RACE
Write by goroutine 8:
  runtime.mapassign1()
      /usr/lib/go/src/pkg/runtime/hashmap.c:1289 +0x0
  github.com/jinzhu/gorm.toSnake()
      /home/edward/gopath/src/github.com/jinzhu/gorm/utils.go:28 +0x2dc
  github.com/jinzhu/gorm.func·001()
      /home/edward/gopath/src/github.com/jinzhu/gorm/model.go:80 +0x255
  gosched0()
      /usr/lib/go/src/pkg/runtime/proc.c:1218 +0x9f

Previous read by goroutine 7:
  runtime.mapaccess1_faststr()
      /usr/lib/go/src/pkg/runtime/hashmap.c:375 +0x0
  github.com/jinzhu/gorm.toSnake()
      /home/edward/gopath/src/github.com/jinzhu/gorm/utils.go:15 +0x74
  github.com/jinzhu/gorm.func·001()
      /home/edward/gopath/src/github.com/jinzhu/gorm/model.go:80 +0x255
  gosched0()
      /usr/lib/go/src/pkg/runtime/proc.c:1218 +0x9f
```