Invoke-MgGraphRequest only supports relative URI when method is GET

[psignoret]

The code which prepends the scheme and host to the request URI is only executed when the method is GET.

Repro:

Invoke-MgGraphRequest `
    -Method POST `
    -Uri "/beta/directoryObjects/getByIds" `
    -Body @{ "ids" = @([Guid]::NewGuid().ToString()); "types" = @("user") }

Invoke-MgGraphRequest : This operation is not supported for a relative URI.
At line:1 char:1
+ Invoke-MgGraphRequest -Method POST -Uri "/beta/directoryObjects/getBy ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Invoke-MgGraphRequest], InvalidOperationException
    + FullyQualifiedErrorId : Microsoft.Graph.PowerShell.Authentication.Cmdlets.InvokeMgGraphRequest

This issue is mentioned in passing in #887, but I think it's deserves a separate issue, as the root cause appears to be unrelated.