Standard filter truncate: truncate_string string coercion

The argument `truncate_string` is now coerced into a string to avoid
`NoMethodError`s. This is mostly for added resiliency. It is doubtful
that someone would actually intent to use a number as truncate string,
but accidentally supplying one is entirely possible.

lib/liquid/standardfilters.rb

@@ -65,9 +65,10 @@ module Liquid
       return if input.nil?
       input_str = input.to_s
       length = Utils.to_integer(length)
-      l = length - truncate_string.length
+      truncate_string_str = truncate_string.to_s
+      l = length - truncate_string_str.length
       l = 0 if l < 0
-      input_str.length > length ? input_str[0...l] + truncate_string : input_str
+      input_str.length > length ? input_str[0...l] + truncate_string_str : input_str
     end
 
     def truncatewords(input, words = 15, truncate_string = "...".freeze)

test/integration/standard_filter_test.rb

@@ -115,6 +115,7 @@ class StandardFiltersTest < Minitest::Test
     assert_equal '...', @filters.truncate('1234567890', 0)
     assert_equal '1234567890', @filters.truncate('1234567890')
     assert_equal "测试...", @filters.truncate("测试测试测试测试", 5)
+    assert_equal '12341', @filters.truncate("1234567890", 5, 1)
   end
 
   def test_split