Fix JSON formatter prefixing old msg/time/level fields

Fixes #88 

r: @Sirupsen cc: @nickvanw 

Wrote a regression test, before fix:

```
cassiopeiae ~/g/s/g/d/c/t/s/g/S/logrus (62b915c...) ➜  go test .
--- FAIL: TestDoubleLoggingDoesntPrefixPreviousFields (0.00s)
        Location:       logrus_test.go:232
	Error:		"map[level:warning msg:omg it is! time:2014-12-09T21:54:19-05:00 fields.level:info fields.msg:looks delicious fields.time:2014-12-09T21:54:19-05:00 context:eating raw fish]" should have 4 item(s), but has 7
	Messages:	should only msg/time/level/context fields

        Location:       logrus_test.go:235
	Error:		Expected nil, but got: "looks delicious"
	Messages:	should not have prefixed previous `msg` entry

FAIL
FAIL	github.com/Sirupsen/logrus	0.008s
```

after fix:

```
cassiopeiae ~/g/s/g/d/c/t/s/g/S/logrus (36a9dbc...) ➜  go test .
ok  	github.com/Sirupsen/logrus	0.008s
```