Get encoding from request headers or page head.

def get_encoding(headers, content):
    """Get encoding from request headers or page head."""
    encoding = None

    content_type = headers.get('content-type')
    if content_type:
        _, params = cgi.parse_header(content_type)
        if 'charset' in params:
            encoding = params['charset'].strip("'\"")

    if not encoding:
        content = utils.pretty_unicode(content[:1000]) if six.PY3 else content

        charset_re = re.compile(r'<meta.*?charset=["\']*(.+?)["\'>]',
                                flags=re.I)
        pragma_re = re.compile(r'<meta.*?content=["\']*;?charset=(.+?)["\'>]',
                               flags=re.I)
        xml_re = re.compile(r'^<\?xml.*?encoding=["\']*(.+?)["\'>]')
        encoding = (charset_re.findall(content) +
                    pragma_re.findall(content) +
                    xml_re.findall(content))
        encoding = encoding and encoding[0] or None

    return encoding