Fix query for new MVW API implementation

Send the request with Content-Type "application/json".

Author: FrankZ85

app/src/main/java/de/christinecoenen/code/zapp/app/mediathek/api/IMediathekApiService.kt

@@ -8,7 +8,7 @@ import retrofit2.http.POST
 
 interface IMediathekApiService {
 
-	@Headers("Content-Type: text/plain")
+	@Headers("Content-Type: application/json")
 	@POST("query")
 	suspend fun listShows(@Body queryRequest: QueryRequest): MediathekAnswer