WebRequest: Change getFullRequestURL() to use local getProtocol()

WebRequest: Change getFullRequestURL() to use local getProtocol()

Previously it relied on wfGetServerUrl to decide the url,
which passed PROTO_CURRENT on to wfExpandUrl(), when then uses
global $wgRequest and checks getProtocol() on that.

For typical web requests, these would be the same object by
reference, but the needless indirection makes the code harder
to reason about and harder to test.

Instead, check this locally and pass on an explicit HTTP or HTTPS
to wfGetServerUrl/wfExpandUrl.

Change-Id: I797bd2f909625536c3af36f015fb2e94bf922ba9