Fix #1264 bolt-http4k fails to parse a request with duplicated header lines (#1266)

Author: seratch

bolt-http4k/src/main/java/com/slack/api/bolt/http4k/Http4kSlackApp.java

@@ -12,6 +12,8 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
@@ -111,9 +113,15 @@ protected com.slack.api.bolt.request.Request<?> toSlackRequest(Request httpReque
         Map<String, List<String>> queryString = toParameters(httpRequest.getUri().getQuery())
                 .stream()
                 .collect(toMap(Pair::component1, it -> singletonList(it.component2())));
-        RequestHeaders headers = new RequestHeaders(httpRequest.getHeaders()
-                .stream()
-                .collect(toMap(Pair::component1, it -> singletonList(it.component2()))));
+
+        Map<String, List<String>> underlyingHeaders = new HashMap<>();
+        for (Pair<String, String> header : httpRequest.getHeaders()) {
+            String name = header.getFirst();
+            List<String> values = underlyingHeaders.getOrDefault(name, new ArrayList<>());
+            values.add(header.getSecond());
+            underlyingHeaders.put(name, values);
+        }
+        RequestHeaders headers = new RequestHeaders(underlyingHeaders);
         return requestParser.parse(SlackRequestParser.HttpRequest.builder()
                 .requestUri(httpRequest.getUri().getPath())
                 .queryString(queryString)

bolt-http4k/src/test/java/test_locally/Http4kSlackAppTest.java

@@ -85,6 +85,8 @@ private static Request slackEventRequest(String requestBody) {
                 .header(X_SLACK_REQUEST_TIMESTAMP, timestamp)
                 .header(X_SLACK_SIGNATURE, signature)
                 .header("Content-Type", "application/json")
+                .header("X-Forwarded-Host", "proxy1.example.com")
+                .header("X-Forwarded-Host", "proxy2.example.com")
                 .query("query", "queryValue")
                 .body(requestBody);
     }
@@ -108,7 +110,7 @@ public void slackEvents() {
                 .header("Content-Type", "application/json; charset=utf-8")
                 .body("{\"text\":\"" +
                         "query: query " +
-                        "headers: content-type,x-slack-request-timestamp,x-slack-signature " +
+                        "headers: x-forwarded-host,content-type,x-slack-request-timestamp,x-slack-signature " +
                         "body: command\\u003decho" +
                         "\"}");