Prepare to publish null safe version (#496)

Author: kevmoo

pubspec.yaml

@@ -1,15 +1,13 @@
 name: http
-version: 0.13.0-nullsafety-dev
+version: 0.13.0-nullsafety.0
 homepage: https://github.com/dart-lang/http
 description: A composable, multi-platform, Future-based API for HTTP requests.
 
 environment:
-  sdk: '>=2.12.0-0 <2.13.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
-# Cannot be published until null safety ships
-publish_to: none
 dependencies:
-  http_parser: ^3.2.0-nullsafety
+  http_parser: ^4.0.0-nullsafety
   meta: ^1.3.0-nullsafety.3
   path: ^1.8.0-nullsafety.1
   pedantic: ^1.10.0-nullsafety
@@ -18,7 +16,6 @@ dev_dependencies:
   test: ^1.16.0-nullsafety.4
 
 dependency_overrides:
+  http_parser: ^4.0.0-nullsafety
   # Because pkg:test does not support pkg:http ^0.13.0
   test: ^1.16.0-nullsafety.5
-  http_parser:
-    git: https://github.com/dart-lang/http_parser