Change content-type check to check contains

So far it was doing an equals check, but a string like "application/json; charset=utf-8"
is similarly valid. And some clients like OkHttp actually automatically add the
charset.

Closes #1152

src/main/java/org/asamk/signal/http/HttpServerHandler.java

@@ -89,7 +89,8 @@ public class HttpServerHandler {
             return;
         }
 
-        if (!"application/json".equals(httpExchange.getRequestHeaders().getFirst("Content-Type"))) {
+        final var contentType = httpExchange.getRequestHeaders().getFirst("Content-Type");
+        if (contentType == null || !contentType.startsWith("application/json")) {
             sendResponse(415, null, httpExchange);
             return;
         }