wsgi.input: Make send_hundred_continue_headers a public API

HTTP specification RFC 7231 (http://tools.ietf.org/html/rfc7231\
responses.

-- RFC 7231 --------------------

6.2.  Informational 1xx

[snip]

   A client MUST be able to parse one or more 1xx responses received
   prior to a final response, even if the client does not expect one.
   A user agent MAY ignore unexpected 1xx responses.
...
--------------------------------

This patch adds a send_hundred_continue_headers() public API method
to wsgi.input, thus allowing WSGI apps to send more than one 100-
continue response.  This does not change existing semantics for the
first 100-continue response on read() or readline().