AER/Default to headers for client identity

[zionts]

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[abernix]

@zionts Does this sound like an appropriate CHANGELOG entry?

A once suggested, slightly incorrect, now tweaked suggestion

When `generateClientInfo` is not used to define the client name, client version and
client reference ID, Apollo Server will default to the values present in the HTTP headers
of the request (`apollographql-client-name`, `apollographql-client-reference-id` and
`apollographql-client-version` respectively) and then, when those headers are not set,
the query extensions' `clientInfo` values. [PR #1960](https://github.com/apollographql/apollo-server/pull/1960)

Update: Done with a slightly different in 8f50045