Release 3.1.2 (#18)

diff --git a/pubspec.yaml b/pubspec.yaml
index 0c209d2..2a5fceb 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: http_parser
-version: 3.1.2-dev
+version: 3.1.2
 author: "Dart Team <misc@dartlang.org>"
 homepage: https://github.com/dart-lang/http_parser
 description: >