Not the servlet spec, but the HTTP spec. RFC 2616 (the HTTP 1.1 spec), Section 5.1.1 has this to say:
An
origin server SHOULD return the status code 405 (Method Not Allowed)
if the method is known by the origin server but not allowed for the
requested resource, and 501 (Not Implemented) if the method is
unrecognized or not implemented by the origin server. The methods GET
and HEAD MUST be supported by all general-purpose servers. All other
methods are OPTIONAL;