RecordedEvent misses "isJson" field

[michael-schnell]

System metadata has a field called "is-json" which currently is not included in "RecordEvent" as all other meta data that has a key in "SystemMetadataKeys". It should also be included in the resulting record event.

[michael-schnell]

Please ignore first (already closed) pull request, it was a mistake. The second one is the right one.

[michael-schnell]

Alternative according to suggestion of @YoEight:

if json then content-type is application/json otherwise it's binary (application/octet-stream)