Specify default Content-Type header for client

The GitHub API expects request bodies to be application/json
Merge pull request #500 from octokit/default-content-type

Specify default Content-Type header for client
v3.3.1
Release 3.3.1
Upgrading guide for v3

Fixes #459
Fix Readme so the upgrade notice is more generic.
Merge pull request #502 from wdperson/readme_update

Fix Readme so the upgrade notice is more generic.
Tweak README to mention semver

[ci skip]