[rubygems/rubygems] Improve TarHeader#calculate_checksum

String#unpack("C*") is equivalent to String#bytes and Array#inject {|a,b| a + b } is equivalent to Array#sum.
This is a minor enhancement in terms of speed and readability.

lib/rubygems/package/tar_header.rb

@@ -208,7 +208,7 @@ def update_checksum
   private
 
   def calculate_checksum(header)
-    header.unpack("C*").inject {|a, b| a + b }
+    header.bytes.sum
   end
 
   def header(checksum = @checksum)