RFR: network: only set Content-type for JSON requests

[Erik Helin]

Hi all,

please review this small patch that makes `RestRequest` only set `Content-type` to `application/json` if the content actually is JSON �� 

Thanks,
Erik

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Commit messages:
 - network: only set Content-type for JSON requests

Changes: https://git.openjdk.java.net/skara/pull/1124/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1124&range=00
  Stats: 12 lines in 1 file changed: 7 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/1124.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1124/head:pull/1124

PR: https://git.openjdk.java.net/skara/pull/1124