Standard filter truncatewords: force truncate_string to string

Currently, `truncatewords` raises a TypeError when the argument
`truncate_string` is an interger. This PR forces string coercion for any
value provided for this argument. Thus,

```ruby
assert_equal 'one two1', @filters.truncatewords("one two three", 2, 1)
```

holds true. Another option would be to raise a `Liquid::ArgumentError`.

What is preferred?

lib/liquid/standardfilters.rb

@@ -76,7 +76,7 @@ module Liquid
       words = Utils.to_integer(words)
       l = words - 1
       l = 0 if l < 0
-      wordlist.length > l ? wordlist[0..l].join(" ".freeze) + truncate_string : input
+      wordlist.length > l ? wordlist[0..l].join(" ".freeze) + truncate_string.to_s : input
     end
 
     # Split input string into an array of substrings separated by given pattern.