fix: fall back to 'charset' of 'content_type' in 'download_as_text' (#326)

Explicit 'encoding' overrides the fallback. Use the 'charset' param of 'content_type', rather than 'content_encoding', which isn't going to be a Unicode -> bytes encoding. Closes #319. Also, rewrap long param descriptions for in-source readability.

Author: tseaver