修复dev存在同名配置时，正确的配置项被覆盖的问题

When the app.conf has [dev] section, the same config item of [dev] section will overwrite the global or other section config item, no matter what `runmode`

for example:
// app.conf

runmode=prod
httpport=80
[dev]
httpport=8080
[prod]
httpport=80

when call to beego.Run, it will use 8080 as the value of beego.HttpPort, not expected 80