Make sure that compression method is STORE for level 0.

Whatever the compression method that is set by the user, if the
compression level is set to 0 (no compression), then the entry should be
STORED. This mimics commandline tool behaviour and matches user
expectations.

lib/zip/entry.rb

@@ -112,7 +112,9 @@ def time=(value)
     end
 
     def compression_method
-      @ftype == :directory ? STORED : @compression_method
+      return STORED if @ftype == :directory || @compression_level == 0
+
+      @compression_method
     end
 
     def compression_method=(method)

test/entry_test.rb

@@ -171,4 +171,25 @@ def test_compression_level_flags
     e_dir = Zip::Entry.new('', 'd/', '', Zip::Entry::DEFLATED, 1)
     assert_equal(0, e_dir.gp_flags & 0b110)
   end
+
+  def test_compression_method_reader
+    [
+      [Zip.default_compression, Zip::Entry::DEFLATED],
+      [0, Zip::Entry::STORED],
+      [1, Zip::Entry::DEFLATED],
+      [9, Zip::Entry::DEFLATED]
+    ].each do |level, method|
+      # Check that the correct method is returned when DEFLATED is specified.
+      entry = Zip::Entry.new('', '', '', Zip::Entry::DEFLATED, level)
+      assert_equal(method, entry.compression_method)
+    end
+
+    # Check that the correct method is returned when STORED is specified.
+    entry = Zip::Entry.new('', '', '', Zip::Entry::STORED, 1)
+    assert_equal(Zip::Entry::STORED, entry.compression_method)
+
+    # Check that directories are always STORED, whatever level is specified.
+    entry = Zip::Entry.new('', 'd/', '', Zip::Entry::DEFLATED, 1)
+    assert_equal(Zip::Entry::STORED, entry.compression_method)
+  end
 end