Site#read_data_file: read CSV's with proper file encoding

Fixes #3451.

lib/jekyll/site.rb

@@ -262,19 +262,22 @@ module Jekyll
         end
       end
     end
-    
+
     # Determines how to read a data file.
     #
     # Returns the contents of the data file.
     def read_data_file(path)
       case File.extname(path).downcase
       when '.csv'
-        CSV.read(path, :headers => true).map(&:to_hash)
+        CSV.read(path, {
+          :headers => true,
+          :encoding => config['encoding']
+        }).map(&:to_hash)
       else
         SafeYAML.load_file(path)
       end
     end
-      
+
     # Read in all collections specified in the configuration
     #
     # Returns nothing.