Add support for strscan 0.7.0 installed with Ruby 2.6

Author: naitoh

lib/rexml/source.rb

@@ -18,6 +18,16 @@ def scan(pattern)
           pattern = /#{Regexp.escape(pattern)}/ if pattern.is_a?(String)
           super(pattern)
         end
+
+        def match?(pattern)
+          pattern = /#{Regexp.escape(pattern)}/ if pattern.is_a?(String)
+          super(pattern)
+        end
+
+        def skip(pattern)
+          pattern = /#{Regexp.escape(pattern)}/ if pattern.is_a?(String)
+          super(pattern)
+        end
       end
     end
     using StringScannerCheckScanString