num_set methods had gone. use set_bits instead

In master branch, `stats` method of native filter is broken due to `num_set` method was replaced by `set_bits` by commit 808ee11bfc250.

Bug verify code and output
```ruby
require 'bloomfilter-rb'

bf = BloomFilter::Native.new
bf.stats

# => undefined method `num_set' for #<CBloomFilter:0x00000000eaaa10> (NoMethodError)
```
so a quick fix to this problem.