wsgi.input: Make send_hundred_continue_headers a public API

Per HTTP RFC 7231 (http://tools.ietf.org/html/rfc7231#section-6.2)
a client is required to be able to process one or more 100 continue
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 sent on read() or readline().
Please rename it to `is_hundred_continue_response_sent` because right now it's easy to misread one letter difference d/t between this attribute and new public method name.