don't add a breaking change to Parser constructor

lib/liquid/parse_context.rb

@@ -30,7 +30,8 @@ module Liquid
     end
 
     def new_parser(input)
-      Parser.new(input, @string_scanner)
+      @string_scanner.string = input
+      Parser.new(@string_scanner)
     end
 
     def new_tokenizer(source, start_line_number: nil, for_liquid_tag: false)

lib/liquid/parser.rb

@@ -2,7 +2,8 @@
 
 module Liquid
   class Parser
-    def initialize(input, string_scanner)
+    def initialize(input)
+      input = input.is_a?(StringScanner) ? input : StringScanner.new(input.to_s)
       @tokens = Lexer.tokenize(input, string_scanner)
       @p      = 0 # pointer to current location
     end