The focus is on methods/behavior implemented by azidentity
package. I didn't aim to hit 100% coverage, mainly because
there are error paths determined by e.g. std http package etc.

Signed-off-by: HandsomeJack <dusek.honza@gmail.com>

Some authorities might require certain headers to be passed.

For example, in our dSTS auth flow, the request form contains
client_info, which needs to be accompanied by X-Client-SKU=MSAL.Go
header, else the API call produces

AADSTS501791: Client_info is only supported for MSAL/ADAL,
please ensure that MSAL/ADAL custom headers are being sent.

The `doForClient` function creates new `runtime.Request` from the incoming
request, but it fails to propagate the respective headers.

This commits is addressing that.

Signed-off-by: HandsomeJack <dusek.honza@gmail.com>