struct marshal gives me extra space before the first field

[csyangchen]

for some struct type, take this one for example:

type T struct {
    A string `json:"a"`
    B []int `json:"b,omitempty"`
}

ffjson MarshalJSON gives me output like this:

{ "a":"123"}
// expected:
{"a":"123"}

the generated code has:

...
buf.WriteString(`{ "a":`)
...

looking into ffjson code, I can see:

...
buf.Write("{ ")
...
buf.Rewind(1)
...

first write the space, then conditionally rewind.

However, the generated code fail on the aforementioned case.

[klauspost]

https://github.com/pquerna/ffjson/blob/master/inception/encoder.go#L110

Pretty sure it is needed though. Probably for this https://github.com/pquerna/ffjson/blob/master/inception/encoder.go#L117

[CAFxX]

Maybe buf.Rewind(1) could become buf.RewindSuffix(",") where RewindSuffix(s) is a function that Rewinds the buffer by len(s) if the buffer ends with s. This way we wouldn't need the extra space in the first place.