fix rubocop Style/CaseEquality cop

.rubocop_rubyzip.yml

@@ -76,10 +76,6 @@ Style/Alias:
 Style/Blocks:
   Enabled: false
 
-# Offense count: 3
-Style/CaseEquality:
-  Enabled: false
-
 # Offense count: 7
 # Configuration parameters: EnforcedStyle, SupportedStyles.
 Style/ClassAndModuleChildren:

lib/zip/entry.rb

@@ -67,7 +67,7 @@ module Zip
       @time               = args[8] || ::Zip::DOSTime.now
 
       @ftype = name_is_directory? ? :directory : :file
-      @extra = ::Zip::ExtraField.new(@extra.to_s) unless ::Zip::ExtraField === @extra
+      @extra = ::Zip::ExtraField.new(@extra.to_s) unless @extra.is_a?(::Zip::ExtraField)
     end
 
     def time
@@ -240,7 +240,7 @@ module Zip
       if extra && extra.bytesize != @extra_length
         raise ::Zip::Error, 'Truncated local zip entry header'
       else
-        if ::Zip::ExtraField === @extra
+        if @extra.is_a?(::Zip::ExtraField)
           @extra.merge(extra)
         else
           @extra = ::Zip::ExtraField.new(extra)

lib/zip/output_stream.rb

@@ -94,7 +94,7 @@ module Zip
       end
       new_entry.comment = comment unless comment.nil?
       unless extra.nil?
-        new_entry.extra = ExtraField === extra ? extra : ExtraField.new(extra.to_s)
+        new_entry.extra = extra.is_a?(ExtraField) ? extra : ExtraField.new(extra.to_s)
       end
       new_entry.compression_method = compression_method unless compression_method.nil?
       init_next_entry(new_entry, level)