Avoid modifying tag when getting description

[simonschmidt]

tags.description was inadvertently modifying the tag data,
causing subsequent uses of that tag to give incorrect results.

Example:

from language_tags import tags

a = len(tags.description('es-419'))
b = len(tags.description('es-419'))

# assert 4 == 7
assert a == b

[coveralls]

Coverage decreased (-1.6%) to 98.098% when pulling c9553f7 on simonschmidt:immutable-description into 2c727b9 on OnroerendErfgoed:master.

[coveralls]

Coverage decreased (-1.6%) to 98.098% when pulling c9553f7 on simonschmidt:immutable-description into 2c727b9 on OnroerendErfgoed:master.

[coveralls]

Coverage decreased (-1.6%) to 98.098% when pulling c9553f7 on simonschmidt:immutable-description into 2c727b9 on OnroerendErfgoed:master.

[coveralls]

Coverage decreased (-1.6%) to 98.098% when pulling c9553f7 on simonschmidt:immutable-description into 2c727b9 on OnroerendErfgoed:master.

[simonschmidt]

I'm not sure why coveralls thinks coverage decreased

[cahytinne]

Strange, I also don't understand why the coverage decreases. The solution looks good!