Forward the preserveHeaderCase argument (dart-lang/http_multi_server#39)

Closes dart-lang/http_multi_server#23

The min SDK constraint is already higher than 2.8 which is the version
which introduced this argument.
diff --git a/pkgs/http_multi_server/CHANGELOG.md b/pkgs/http_multi_server/CHANGELOG.md
index 32eca00..a6ec3d9 100644
--- a/pkgs/http_multi_server/CHANGELOG.md
+++ b/pkgs/http_multi_server/CHANGELOG.md
@@ -1,4 +1,6 @@
-## 3.1.1-dev
+## 3.2.0
+
+* Honor the `preserveHeaderCase` argument to `MultiHeaders.set` and `.add`.
 
 ## 3.1.0
 
diff --git a/pkgs/http_multi_server/lib/src/multi_headers.dart b/pkgs/http_multi_server/lib/src/multi_headers.dart
index 92bfd64..c5ee3d2 100644
--- a/pkgs/http_multi_server/lib/src/multi_headers.dart
+++ b/pkgs/http_multi_server/lib/src/multi_headers.dart
@@ -96,7 +96,7 @@
   @override
   void add(String name, Object value, {bool preserveHeaderCase = false}) {
     for (var headers in _headers) {
-      headers.add(name, value);
+      headers.add(name, value, preserveHeaderCase: preserveHeaderCase);
     }
   }
 
@@ -128,7 +128,7 @@
   @override
   void set(String name, Object value, {bool preserveHeaderCase = false}) {
     for (var headers in _headers) {
-      headers.set(name, value);
+      headers.set(name, value, preserveHeaderCase: preserveHeaderCase);
     }
   }
 
diff --git a/pkgs/http_multi_server/pubspec.yaml b/pkgs/http_multi_server/pubspec.yaml
index c03d358..02f9ebe 100644
--- a/pkgs/http_multi_server/pubspec.yaml
+++ b/pkgs/http_multi_server/pubspec.yaml
@@ -1,5 +1,5 @@
 name: http_multi_server
-version: 3.1.1-dev
+version: 3.2.0
 
 description: >-
   A dart:io HttpServer wrapper that handles requests from multiple servers.