VectorFeatureWriterHandler: Handle null and non-string properties

[jampukka]

Handle non-string and null property values, avoid errors such as:

org.json.JSONException: JSONObject["aloitusvuosi"] is not a string (class java.lang.Integer : 2024).
org.json.JSONException: JSONObject["asuntoalat"] is not a string (class org.json.JSONObject$Null : null).
        at org.json.JSONObject.wrongValueFormatException(JSONObject.java:3016)
        at org.json.JSONObject.getString(JSONObject.java:943)
        at fi.nls.oskari.control.feature.AbstractFeatureHandler.getFeature(AbstractFeatureHandler.java:171)
        at fi.nls.oskari.control.feature.VectorFeatureWriterHandler.handlePost(VectorFeatureWriterHandler.java:71)