常见的http请求头
ACCEPT = 'Accept'
ACCEPT_CHARSET = 'Accept-Charset'
ACCEPT_ENCODING = 'Accept-Encoding'
ACCEPT_LANGUAGE = 'Accept-Language'
ACCEPT_DATETIME = 'Accept-Datetime'
AUTHORIZATION = 'Authorization'
CACHE_CONTROL = 'Cache-Control'
CONNECTION = 'Connection'
COOKIE = 'Cookie'
CONTENT_LENGTH = 'Content-Length'
CONTENT_MD5 = 'Content-MD5'
CONTENT_TYPE = 'Content-Type'
DATE = 'Date'
EXPECT = 'Expect'
FROM = 'From'
HOST = 'Host'
IF_MATCH = 'If-Match'
IF_MODIFIED_SINCE = 'If-Modified-Since'
IF_NONE_MATCH = 'If-None-Match'
IF_RANGE = 'If-Range'
IF_UNMODIFIED_SINCE = 'If-Unmodified-Since'
MAX_FORWARDS = 'Max-Forwards'
ORIGIN = 'Origin'
PRAGMA = 'Pragma'
PROXY_AUTHORIZATION = 'Proxy-Authorization'
RANGE = 'Range'
REFERER = 'Referer'
TE = 'TE'
USER_AGENT = 'User-Agent'
UPGRADE = 'Upgrade'
VIA = 'Via'
WARNING = 'Warning'
TRAILER = 'trailer'