[INTERNAL] util/fetch: add protocol back to protocol-relative URL string
[INTERNAL] util/fetch: add protocol back to protocol-relative URL string

- Cloud portal interceptor can't handle protocol-relative URLs (the ones
  that start directly with "//" without having a preceeding protocol).
  The missing protocol was added within jQuery.ajax implementation and we
  have to do the same in fetch.js.

Change-Id: I9936183ef9701fb13327384a2533f951b5e8e996

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5