liquid_tags.* plugins are not unicode_aware

For some of them, like youtube or vimeo, this makes no difference, but for img it does *(you want to be able to write titles in your language)* and for include_code it presents a more complicated problem -- the source code file might not actually be unicode in the first place. *(There are certain cases where this would be the norm.)*

This commit fixes both img and include_code. Tested locally, seems to work.