Add 'easyinvoice-source' header to the create function

[dveldhoen]

To be able to identify which package is being used to generate invoices we can add a header to the POST request which the backend uses to identify the traffic source.

The header key should be: easyinvoice-source
The header value should be: pip

The backend will recognize this header automatically and log every created invoice in the database with pip as source.