Prepare to publish (#24)

We want a forwards compatible package available. The SDK change has been
merged and the name is finalized.

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6bce625..eed6fab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 2.2.0-dev
+## 2.2.0
 
 - Preparation for [HttpHeaders change]. Update signature of `MultiHeaders.add()`
   and `MultiHeaders.set()` to match new signature of `HttpHeaders`. The

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index ad3b92a..9f84247 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: http_multi_server
-version: 2.2.0-dev
+version: 2.2.0
 publish_to: none # https://github.com/dart-lang/http_multi_server/issues/23
 
 description: >-