Invalid ContentType header due to duplicate application/json value

[cDanowski]

In some deployment scenarios (where communication between importer and management component happened via Apache Reverse Proxy), the issue occurred that the ContentType header when contained the value "application/json, application/json".

This is invalid because of the comma "," separator und duplicate value.

While in similar deployment scenarios the code was working just fine (even though Reverse Proxy was in the middle as well) one solution is to remove the CONTENT-TYPE default header as implemented in kommonitor-importer-app/src/main/java/org/n52/kommonitor/importer/KommonitorImporterConfiguration.java

The fix is alredy applied in commit 9734987

Up to now no other problem has occurred as a consequence of the adjustment.