Invalid json when using only an objet

[IxDay]

When I use zerolog with only an object the json output is incorrect.

import (
    "os"

    "github.com/rs/zerolog"
)
type foo struct{}

func (f foo) MarshalZerologObject(e *zerolog.Event) { e.Str("bar", "baz") }

func main() {
    logger := zerolog.New(os.Stdout).With().Object("foo", foo{}).Logger()   
    logger.Info().Msg("qux")
}

This will output:

{"level":"info",,"foo":{"bar":"baz"},"message":"qux"}
               ^-- invalid double comma

When pushing any additional field other than object:

func main() {
    logger := zerolog.New(os.Stdout).With().Str("bar", "foo").Object("foo", foo{}).Logger()   
    logger.Info().Msg("qux")
}

I get a valid json:

{"level":"info","bar":"foo","foo":{"bar":"baz"},"message":"qux"}

[IxDay]

The bug is actually triggered if Object is the first pushed to the logger, i.e:

logger := zerolog.New(os.Stdout).With().Object("foo", foo{}).Str("bar", "foo").Logger() 
logger.Info().Msg("qux")

Problem seems to come from AppendObjectData I did a quick fix which seems to be working, however, code is not really elegant:

func (Encoder) AppendObjectData(dst []byte, o []byte) []byte {
	if o[0] == '{' {
		if len(dst) == 0 {
			o = o[1:]
		} else {
			o[0] = ','
		}
	} else if len(dst) > 1 {
		dst = append(dst, ',')
	}
	return append(dst, o...)
}

[rs]

With some comment is should be ok. If you can add unit test too, it would be awesome (considering you are willing to send a PR).

[IxDay]

I can do that :) give me a bit of time to implement it

[IxDay]

Is there anything I need to add?