instance method determine_install_method

# File railties/lib/rails/commands/plugin.rb, line 389
      def determine_install_method
        best = @base_command.environment.best_install_method
        @method = :http if best == :http and @method == :export
        case
        when (best == :http and @method != :http)
          msg = "Cannot install using subversion because `svn' cannot be found in your PATH"
        when (best == :export and (@method != :export and @method != :http))
          msg = "Cannot install using #{@method} because this project is not under subversion."
        when (best != :externals and @method == :externals)
          msg = "Cannot install using externals because vendor/plugins is not under subversion."
        end
        if msg
          puts msg
          exit 1
        end
        @method
      end