fix: fall back to 'charset' of 'content_type' in 'download_as_text' (googleapis#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 googleapis#319.

Also, rewrap long param descriptions for in-source readability.

Author: tseaver