Prepare to publish with more doc fixes (#349)

The stale docs are getting noticed: #164
diff --git a/pubspec.yaml b/pubspec.yaml
index 270e395..fa497de 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: http
-version: 0.12.0+3-dev
+version: 0.12.0+3
 author: "Dart Team <misc@dartlang.org>"
 homepage: https://github.com/dart-lang/http
 description: A composable, multi-platform, Future-based API for HTTP requests.