concurrency issue on Logger

[goenning]

fatal error: concurrent map iteration and map write

goroutine 10516 [running]:
runtime.throw(0x157a73a, 0x26)
	/usr/local/go/src/runtime/panic.go:616 +0x81 fp=0xc420841d40 sp=0xc420841d20 pc=0x102c801
runtime.mapiternext(0xc420841e48)
	/usr/local/go/src/runtime/hashmap.go:747 +0x55c fp=0xc420841dd0 sp=0xc420841d40 pc=0x100aa1c
runtime.mapiterinit(0x14f1e20, 0xc4204346c0, 0xc420841e48)
	/usr/local/go/src/runtime/hashmap.go:737 +0x1f1 fp=0xc420841df8 sp=0xc420841dd0 pc=0x100a3d1
github.com/getfider/fider/app/pkg/log.Props.Merge(0xc4204346c0, 0xc4204348a0, 0xc4204347b0)
	/Users/goenning/Documents/workspace/go-workspace/src/github.com/getfider/fider/app/pkg/log/log.go:97 +0x19d fp=0xc420841f18 sp=0xc420841df8 pc=0x12588bd
github.com/getfider/fider/app/pkg/log/database.(*Logger).log.func1()
	/Users/goenning/Documents/workspace/go-workspace/src/github.com/getfider/fider/app/pkg/log/database/logger.go:124 +0x67 fp=0xc420841fe0 sp=0xc420841f18 pc=0x12bbf57
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:2361 +0x1 fp=0xc420841fe8 sp=0xc420841fe0 pc=0x10585c1
created by github.com/getfider/fider/app/pkg/log/database.(*Logger).log
	/Users/goenning/Documents/workspace/go-workspace/src/github.com/getfider/fider/app/pkg/log/database/logger.go:156 +0x134