Make sure APIGatewayProxyResponseEvent gets a mutable header map

Author: mduesterhoeft

router/src/main/kotlin/io/moia/router/RequestHandler.kt

@@ -204,7 +204,7 @@ abstract class RequestHandler : RequestHandler<APIGatewayProxyRequestEvent, APIG
         }.let { finalContentType ->
             APIGatewayProxyResponseEvent()
                 .withStatusCode(response.statusCode)
-                .withHeaders(response.headers + ("Content-Type" to finalContentType.toString()))
+                .withHeaders(response.headers.toMutableMap().apply { put("Content-Type", finalContentType.toString()) })
                 .withBody(response.body?.let {
                     serializationHandlerChain.serialize(finalContentType, it as Any)
                 })

router/src/test/kotlin/io/moia/router/RequestHandlerTest.kt

@@ -539,7 +539,7 @@ class RequestHandlerTest {
         private val incrementingFilter = Filter { next ->
             { request ->
                 filterInvocations += 1
-                next(request).apply { headers = headers + ("header" to "value") }
+                next(request).apply { withHeader("header", "value") }
             }
         }
         override val router = router {