Release 2.3.0 for Dart 2.8

• Fix 85 The method '_ReadOnlyHeaders.add' has fewer named arguments than those of overridden method 'HttpHeadersWrapper.add [Dart 2.8]

Release 2.2.2+1

• Limit Dart SDK version for Dart 2.8 version breaking change.
• Add links to github repo in pubspec.yaml.