do not authenticate user on xmlprc setting #17

first of all it is to early in the wp process. the user is not setup there.
furthermore the authentication is useless. the xmlrpc interface performs an
authentication on its own.
place xmlrpc_enabled filter only on xmlrpc requests

the filter on xmlrpc_enabled is evaluated during the authentication process
of the xmlrpc server. the constant XMLRPC_REQUEST is allready set there so
it can be a valid indicator for the filter to be set.