Reduce allocations in truncatewords

I don't know if this is a bottleneck IRL, but while doing some
performance testing I noticed that this method allocates a bunch of
objects and I decided to fix it.

Here is the benchmark I used:

```ruby
require_relative 'theme_runner'

Liquid::Template.error_mode = ARGV.first.to_sym if ARGV.first
profiler = ThemeRunner.new

def count_alloc
  before = GC.stat(:total_allocated_objects)
  yield
  GC.stat(:total_allocated_objects) - before
end

profiler.render # heat
p count_alloc { profiler.render }
p count_alloc { profiler.render }
p count_alloc { profiler.render }
```

Before this change:

```
[aaron@tc-lan-adapter ~/g/liquid (master)]$ ruby -I lib performance/run_once.rb
15753
15750
15752
```

After this change:

```
[aaron@tc-lan-adapter ~/g/liquid (master)]$ ruby -I lib performance/run_once.rb
14015
14010
14011
```

About an 11% reduction in allocations for this test.  I also added some
tests around the current behavior of the method so we can be more sure
the replacement behaves the same way

lib/liquid/standardfilters.rb

@@ -89,13 +89,22 @@ module Liquid
 
     def truncatewords(input, words = 15, truncate_string = "...")
       return if input.nil?
-      wordlist = input.to_s.split
-      words    = Utils.to_integer(words)
+      words = Utils.to_integer(words)
 
-      l = words - 1
-      l = 0 if l < 0
+      l = words
+      l = 1 if l <= 0
 
-      wordlist.length > l ? wordlist[0..l].join(" ").concat(truncate_string.to_s) : input
+      # Scan for non-space characters followed by one or more space characters
+      # `l` times.  Also ignore leading whitespace
+      str = input[/\A[ ]*(?:[^ ]*[ ]+){#{l}}/]
+
+      if str
+        str.strip!                # Remove trailing space
+        str.gsub!(/[ ]{2,}/, " ") # Shrink multiple spaces to one space
+        str.concat(truncate_string.to_s)
+      else
+        input
+      end
     end
 
     # Split input string into an array of substrings separated by given pattern.