HTTP auth header parser

Parse an HTTP Authorization / WWW-Authenticate header into its scheme and parameters. The header is 'Scheme param1=value1, param2="value 2", ...'. The scheme is the first whitespace-delimited token. Parameters follow as comma-separated key=value pairs; values may be bare tokens or double-quoted strings, and a quoted value may itself contain commas (which must NOT be treated as parameter separators). Strip surrounding quotes from values. If there are no parameters (e.g. 'Bearer abc123'), return an empty params dict. Return [scheme, params_dict].