Shell::windows? function should include check for mingw

[mwolfe38]

Recently the installer for ruby 1.9.1 was changed such that the binaries are compiled with mingw rather than the mswin compiler that was used before.. This changes the platform string such that there is no mswin portion anymore and makes your windows check invalid.
Thus, if you could change it as follows:

def windows?
RUBY_PLATFORM =~ /mswin|mingw/
end

It will fix lots of issues that are sure to happen as more people begin installing the newly compiled windows version

[bosko]

Event better would be:

def windows?
   ::RbConfig::CONFIG['host_os'] =~ /mswin|mingw/
end