fix the case when toJSON() returns a Buffer

[chatziko]

has-binary assumes that toJSON() returns an object, but this is not necessary.
It incorrectly returns false for objects whose toJSON() returns a Buffer, eg:

{ a: 'a', toJSON: function() { return new Buffer('abc') } }

Returning Buffer from toJSON is useful for classes who want to serialize themselves in binary when sent over socket.io.

The fix is simple, I also added a test.

[darrachequesne]

Merged as #6, thanks!

[chatziko]

Thanks for the merge!

Maybe it's a detail, but if the aim of has-binary is to match JSON.stringify, then toJSON() should not be called again in the object returned by toJSON(). In other words:

> var o = { toJSON: function() { return this } }
> JSON.stringify(o)
'{}'
> hasBinary(o)
RangeError: Maximum call stack size exceeded

That's why my fix was a bit more complicated than a recursive call.

[darrachequesne]

@chatziko #7 what do you think?

[chatziko]

Looks good.

(looking forward to a release to stop maintaining my own fork :D)