Codecs charset support

- fixed codecs/multiline to use Charset#convert and added specs. 
- added encoding specs to codecs/plain 

I started with codecs/plain to add specs to a correctly implemented codec, then moved on to refactor multiline and also add encoding specs.

These specs are clearly redondant to the specs we have in util/charset_spec - we could think about DRYing this somehow. 

Let me know what you think, do not merge just yet, I will finish reviewing all codecs.