fix: content type for v1 payload should be retrieved from multi value headers (#1193)

Author: deki

aws-serverless-java-container-springboot3/src/main/java/com/amazonaws/serverless/proxy/spring/AwsSpringHttpProcessingUtils.java

@@ -126,7 +126,7 @@ private static HttpServletRequest generateRequest1(String request, Context lambd
 		}
         populateContentAndContentType(
                 v1Request.getBody(),
-                v1Request.getHeaders().get(HttpHeaders.CONTENT_TYPE),
+                v1Request.getMultiValueHeaders().getFirst(HttpHeaders.CONTENT_TYPE),
                 v1Request.isBase64Encoded(),
                 httpRequest
         );