Implemented Encoding::Converter#putback.
Untag passing specs.
Add specs on EncodingConverter#primitive_convert on clearing the source buffer.
Fix EncodingConverter#primitive_convert on clearing the source buffer.
Update existing usage of EncodingConverter#primitive_convert.
Merge pull request #2540 from razielgn/primitive_convert

EncodingConverter#primitive_convert clears the source buffer.