Allow creating of Buffer with source

lib/parser/source/buffer.rb

@@ -102,7 +102,7 @@ def self.reencode_string(input)
         end
       end
 
-      def initialize(name, first_line = 1)
+      def initialize(name, first_line = 1, source: nil)
         @name        = name.to_s
         @source      = nil
         @first_line  = first_line
@@ -116,6 +116,8 @@ def initialize(name, first_line = 1)
         # Cache for fast lookup
         @line_for_position   = {}
         @column_for_position = {}
+
+        self.source = source if source
       end
 
       ##

test/test_source_buffer.rb

@@ -20,6 +20,9 @@ def test_initialize
 
     buffer = Parser::Source::Buffer.new('(string)', 5)
     assert_equal 5, buffer.first_line
+
+    buffer = Parser::Source::Buffer.new('(string)', source: '2+2')
+    assert_equal '2+2', buffer.source
   end
 
   def test_source_setter