No need for featureNS and featureType to be quoted

In the GML format, [`featureNS` and `featureType`](https://github.com/openlayers/ol3/blob/ad1f255211aeadc522ebb9edbfcf27dfc4a89015/src/ol/format/gmlformat.js#L1068-L1071) should not be quoted.  We would only need to quote these if they were element names or if we were passing the context object to users.

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