Fix unitialized instance var warnings

lib/xlsxtream/io/directory.rb

@@ -5,6 +5,7 @@ module IO
     class Directory
       def initialize(path)
         @path = Pathname(path)
+        @file = nil
       end
 
       def <<(data)
@@ -19,7 +20,7 @@ def add_file(path)
       end
 
       def close
-        @file.close if @file.respond_to? :close
+        @file.close if @file
       end
     end
   end

lib/xlsxtream/io/stream.rb

@@ -3,6 +3,7 @@ module IO
     class Stream
       def initialize(stream)
         @stream = stream
+        @path = nil
       end
 
       def <<(data)

lib/xlsxtream/io/zip_tricks.rb

@@ -5,6 +5,7 @@ module IO
     class ZipTricks
       def initialize(body)
         @streamer = ::ZipTricks::Streamer.new(body)
+        @wf = nil
       end
 
       def <<(data)

lib/xlsxtream/workbook.rb

@@ -48,8 +48,10 @@ def initialize(output, options = {})
         @file = File.open(output, 'wb')
         @io = IO::ZipTricks.new(@file)
       elsif output.respond_to? :add_file
+        @file = nil
         @io = output
       else
+        @file = nil
         @io = IO::ZipTricks.new(output)
       end
       @sst = SharedStringTable.new