HTTP and character encodings

Hi,

tl;dr: I'd like to remove the String instances from the HTTP package.

The HTTP library is overloaded on the type for request and response
bodies; there are instances for String and both strict and lazy Bytestrings.

Unfortunately, the String instance is rather broken. A String ought to
represent Unicode data, but the HTTP wire format is bytes, and HTTP
makes no attempt to handle encoding.

In particular uploaded data (e.g. in POSTs) gets silently truncated and
downloaded data is improperly embedded as one byte per character no
matter what encoding the server advertises in the Content-Type header.
(https://github.com/haskell/HTTP/issues/28)

I've spent a while investigating the option of making HTTP encode and
decode Strings appropriately, but my tentative conclusion is that it's
too hard:

- on upload we'd have to pick an encoding by default - probably UTF-8 -
and also add it to the Content-Type header which may involve messing
with any header supplied by the user. If the user supplied a different
encoding in Content-Type then we probably would need to notice and
respect that.

- on upload Content-Length may also need to be managed somehow.

- on download we'd need to be able to handle at least common encodings
that the server might send, but on Windows even common encodings like