Adds support for a sub vendor app id option to be sent to server among request headers as X-Vendor-App-Id
Added tests for the `X-Vendor-App-Id` header and cleaned up other tests
Merge pull request #1 from mobilerider/feature/allow-sub-vendor-app-header

Adds support for a sub vendor app id option, sent as the `X-Vendor-App-Id` request header.