Fix loading certificate from .pfx file

If you tried this
 Use-Icinga; Get-IcingaSSLCertForSocket -CertFile "C:/my/cert.pfx"

the former code expanded like
 $FileType = Get-Item -Path "C:/my/cert.pfx";
 if ("cert.pfx" -eq '.pfx') { ... not reached ... }

and thus did not load the certificate file.

Now use GetExtension instead of Get-Item to make the .pfx check work.

lib/webserver/Get-IcingaSSLCertForSocket.psm1

@@ -10,8 +10,7 @@ function Get-IcingaSSLCertForSocket()
     # to get a proper certificate
     if ([string]::IsNullOrEmpty($CertFile) -eq $FALSE) {
         if ((Test-Path $CertFile)) {
-            $FileType = Get-Item -Path $CertFile;
-            if ($FileType -eq '.pfx') {
+            if ([IO.Path]::GetExtension($CertFile) -eq '.pfx') {
                 return (New-Object Security.Cryptography.X509Certificates.X509Certificate2 $CertFile);
             } else {
                 return ConvertTo-IcingaX509Certificate -CertFile $CertFile;