Use warn instead of puts for messages from the library code.

`warn` has the advantage of easily being disabled by, the `-W0` interpreter
flag or setting $VERBOSE to nil.

lib/zip/entry.rb

@@ -163,7 +163,7 @@ def next_header_offset #:nodoc:all
     # is passed.
     def extract(dest_path = nil, &block)
       if dest_path.nil? && !name_safe?
-        puts "WARNING: skipped #{@name} as unsafe"
+        warn "WARNING: skipped #{@name} as unsafe"
         return self
       end
 
@@ -614,7 +614,7 @@ def create_file(dest_path, _continue_on_exists_proc = proc { Zip.continue_on_exi
               if ::Zip.validate_entry_sizes
                 raise ::Zip::EntrySizeError, message
               else
-                puts "WARNING: #{message}"
+                warn "WARNING: #{message}"
                 warned = true
               end
             end
@@ -642,7 +642,7 @@ def create_directory(dest_path)
     def create_symlink(dest_path)
       # TODO: Symlinks pose security challenges. Symlink support temporarily
       # removed in view of https://github.com/rubyzip/rubyzip/issues/369 .
-      puts "WARNING: skipped symlink #{dest_path}"
+      warn "WARNING: skipped symlink #{dest_path}"
     end
 
     # apply missing data from the zip64 extra information field, if present

lib/zip/extra_field/generic.rb

@@ -16,7 +16,7 @@ def initial_parse(binstr)
         # If nil, start with empty.
         return false
       elsif binstr[0, 2] != self.class.const_get(:HEADER_ID)
-        $stderr.puts 'Warning: weired extra feild header ID. skip parsing'
+        warn 'Warning: weired extra feild header ID. skip parsing'
         return false
       end
       [binstr[2, 2].unpack('v')[0], binstr[4..-1]]

lib/zip/input_stream.rb

@@ -103,7 +103,7 @@ def open(filename_or_io, offset = 0, decrypter = nil)
       end
 
       def open_buffer(filename_or_io, offset = 0)
-        puts 'open_buffer is deprecated!!! Use open instead!'
+        warn 'open_buffer is deprecated!!! Use open instead!'
         open(filename_or_io, offset)
       end
     end