Land #11174, Fix platform bug when upgrade shell.

The platform on windows powershell should be 'win', rather than
'windows', this bug leads to failure when upgrade powershell session
to meterpreter.

modules/post/multi/manage/shell_to_meterpreter.rb

@@ -73,7 +73,7 @@ class MetasploitModule < Msf::Post
 
     # Handle platform specific variables and settings
     case session.platform
-    when 'windows'
+    when 'windows', 'win'
       platform = 'windows'
       payload_name = 'windows/meterpreter/reverse_tcp'
       lplat = [Msf::Platform::Windows]