Be less picky about line endings in headers in httpclient.

HTTP requires that lines end with \r\n, but some buggy servers (including
news.ycombinator.com) just use \n.  Libcurl tolerates this and sends the
line as-is to the header callback, so we need to be prepared to handle
either form.

tornado/httpclient.py

@@ -579,10 +579,12 @@ def _curl_setup_request(curl, request, buffer, headers):
 
 
 def _curl_header_callback(headers, header_line):
+    # header_line as returned by curl includes the end-of-line characters.
+    header_line = header_line.strip()
     if header_line.startswith("HTTP/"):
         headers.clear()
         return
-    if header_line == "\r\n":
+    if not header_line:
         return
     headers.parse_line(header_line)