Fix UTF-8 encoding in gpgdata write callback

[ice799]

This change associates the buffer allocated in the gpgme_data write
callback with the UTF-8 encoding. If there is a configured internal
encoding (e.g. Encoding.default_internal), then the string is exported
to that encoding.

[ice799]

This fixes this bug: #113

Probably would want to consider in the future unifying the encoding logic used in utf8_str_new and the logic added in this PR to work the same way -- users would probably be happy if they could set their own internal encoding that would affect both code paths.

[coveralls]

Coverage increased (+0.06%) to 90.518% when pulling 5eb4513 on computology:utf8_cb into 387c775 on ueno:master.

[ice799]

This fixes #113

[tommeier]

Hi 👋 any news on getting this merged in?

[djberg96]

@ice799 @tommeier I think #208 should've fixed this.