fix non-ASCII header exception in Python 3

[BennyThink]

Hi there!

This Pull Request contains fix for non-ASCII(emoji, CJK characters, etc.) http headers in Python 3.
For example, refer to the following code:

def get(self):
    file_name, file_content = 'hello😁.txt', 'how are you'
    self.set_header("Content-Type", "application/bin; charset='utf-8'")
    self.set_header("Content-Disposition", "attachment; filename=%s" % file_name)
    self.write(file_content)

Above code will trigger browser to download hello😁.txt instead of open it.
However, above code would raise such exception:
UnicodeEncodeError: 'latin-1' codec can't encode characters in position 45-46: ordinal not in range(256)
And also, non-ASCII header set in any field would raise this exception,
for example self.set_header("laugh😁", "emoji")

The error occurs in the L385 of http1connection.py:
lines.extend(l.encode('latin1') for l in header_lines)
Of course encoding an emoji or CJK in latin1 would throw an exception.

The solution varies, whether url_escape(file_name) when we programs or fix the implementation in http1connection.py

Though according to RFC7230 section 3.2.4:

Historically, HTTP has allowed field content with text in the
ISO-8859-1 charset [ISO-8859-1], 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 [USASCII].
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.

HTTP Header should be limited to ASCII value.

However, this PR seems like a temporary solution since the Python 2 version just send the header in raw.

Anyway, thanks for your precious time in reviewing this.

[bdarnell]

The RFC is clear here: headers must be sent in latin1, or, for specific headers that permit this, RFC2047 encoding (the content-disposition header is one that permits RFC2047 encoding). The python 3 behavior is correct. If we change anything, it should be to make python 2 reject these invalid headers in the same way python 3 does. (and maybe some helpers to make it easier to work with RFC2047)