Fix debug string in ParseError message

Author: rmosolgo

lib/graphql/language/lexer.rb

@@ -130,7 +130,7 @@ def debug_token_value(token_name)
         elsif token_name == :STRING
           string_value
         elsif @scanner.matched_size.nil?
-          @scanner.peek(1)
+          @string.byteslice(@scanner.pos - 1, 1)
         else
           token_value
         end

spec/graphql/language/parser_spec.rb

@@ -235,6 +235,19 @@
     assert_equal expected_msg, err.message
   end
 
+  it "shows the right character in error messages" do
+    err = assert_raises(GraphQL::ParseError) {
+      GraphQL.parse("query # comment\n ~ { a b }")
+    }
+    expected_msg = if USING_C_PARSER
+      "syntax error, unexpected invalid token (\"~\"), expecting LCURLY at [2, 2]"
+    else
+      "Expected NAME, actual: UNKNOWN_CHAR (\"~\") at [2, 2]"
+    end
+
+    assert_equal expected_msg, err.message
+  end
+
   it "can reject name start at the end of numbers" do
     prev_reject_numers_followed_by_names = GraphQL.reject_numbers_followed_by_names
     GraphQL.reject_numbers_followed_by_names = false