Handle relative URLs correctly

Return the connection string instead of the connection URL

This allows us to construct relative URLs by appending strings instead of using the relative URL method, which doesn't work if the base URL is a URL as opposed to just a hostname.

e-mission/e-mission-docs#732 (comment)

Related updates:

• https://github.com/e-mission/cordova-server-communication/releases/tag/v1.2.4
• https://github.com/e-mission/cordova-server-sync/releases/tag/v1.2.9