After upgrading to iOS SalesForceSDKCore v5.2.0 now my POST API does not work.
It's givingthis error, 
errorCode = "INVALID_CONTENT_TYPE";
message = "Missing Content-Type HTTP header";

The query parameters are like this:

Status=Planned&aUserID=0055C000000ExaoQAC&endDate=1504029480000&startDate=1504027680000&accID=001U000000UQTZcIAP&IsPrivate=false&EventObjective=Development&CancellationReason=&AlldayEvent=false&FixedEvent=true&EventType=Visit

Any idae?