Fix: keep original jsoncore config

[jmarkerink]

This PR is to honor the initial configured JsonCore setting

I excluded the jackson libraries from the classpath and configured the JsonUnflattener to use the GsonJsonCore, but I got the next exception when using:

com/fasterxml/jackson/core/JsonProcessingException
java.lang.NoClassDefFoundError: com/fasterxml/jackson/core/JsonProcessingException
	at com.github.wnameless.json.unflattener.JsonUnflattener.<init>(JsonUnflattener.java:124)
	at com.github.wnameless.json.unflattener.JsonUnflattener.newJsonUnflattener(JsonUnflattener.java:114)
	at com.github.wnameless.json.unflattener.JsonUnflattener.setUnflattenedValue(JsonUnflattener.java:533)
	at com.github.wnameless.json.unflattener.JsonUnflattener.unflatten(JsonUnflattener.java:407)