Non-ASCII headers are handled incorrectly by CurlAsyncHTTPClient

[Sjord]

If a server returns a non-ASCII character in a header, the curl client should still handle the response, but it raises an UnicodeDecodeError instead.

Code:

yield CurlAsyncHTTPClient().fetch('https://httpbin.org/response-headers?X-Foo=bl%C3%A9')

Got:

Traceback (most recent call last):
  File "/home/sjoerd/.virtualenvs/protomon/lib/python3.4/site-packages/tornado/curl_httpclient.py", line 453, in _curl_header_callback
    header_line = native_str(header_line)
  File "/home/sjoerd/.virtualenvs/protomon/lib/python3.4/site-packages/tornado/escape.py", line 222, in to_unicode
    return value.decode("utf-8")
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 9: invalid continuation byte

RFC 7230 says:

Historically, HTTP has allowed field content with text in the ISO-8859-1 charset, supporting other charsets only through use of RFC2047 encoding. In practice, most HTTP header field values use only a subset of the US-ASCII charset. Newly defined header fields SHOULD limit their field values to US-ASCII octets. A recipient SHOULD treat other octets in field content (obs-text) as opaque data.