Double-quotes need to be escaped in NQuads serialization

[frmichel]

Hi Markus,

When a string in the JSON-LD document contains escaped double-quotes, like in "Here is a \"string\n", then the double quotes are no longer escaped in the NQuads serialization.
I fixed it in https://github.com/lanthaler/JsonLD/blob/master/NQuads.php by changing this line

$result` .= '"' . $quad->getObject()->getValue() . '"';

into

$result .= '"' . str_replace ( '"', '\"', $quad->getObject()->getValue() ). '"';

I did not test other serializations but it is likely that the same issue occurs.

Regards,
Franck.