fixed JS mapping header serialization to string for array headers

[thjaeckle]

This fixes an issue about JavaScript mapping in Ditto connections which use Json arrays as value inside headers.

Right now, this leads to e.g. the following follow-up exception (trying to parse requested-acks header as array):

The value 'org.mozilla.javascript.NativeArray@e90b588' of the header 'requested-acks' is not a valid JSON array. Verify that the value of the header 'requested-acks' is a valid 'JSON array' and try again.

So arrays were not correctly serialized, but their toString() on NativeArray (from Rhino library) was invoked, which just displays the default toString() implementation.

This fix will transform the Rhino NativeObject and NativeArray to ditto-json JsonObject and JsonArray and will use the buit-in formatAsString() for header values.

[thjaeckle]

@alstanchev could you have a look, please?

[alstanchev]

LGTM 👍