Don't strip characters that match the first codepoints in multi-codepoint emoji in Rumoji.encode_io

Running `Rumoji.encode_io` on a stream with characters that match the first codepoints in multi-codepoint emoji causes those characters to get stripped:

``` ruby
io = StringIO.new("i like #hashtags and 1direction")
Rumoji.encode_io(io).string
# => "i like hashtags and direction"
```

This branch fixes it.