If using jsonpath, properties aren't properly marked as existing

[neonerd]

When using jsonpath, the library still uses key in body to determine whether given key exists.

return new Validator(this, key, getValue(body,key,transFn), key in body , body);

Therefore, with a body like this:

{"foo":{"bar":"hello world"}}

Following code will return an error:

this.checkBody('/foo/bar', true).first().exist()

Because the property is marked as exists:false.

[neonerd]

Wrote a fix in #20

[neonerd]

Merged.