According to HTTP/1.1 specification, the server should return 501 in case of the method being completely unknown to the origin server. The Undertow returns in such case 405, which is incorrect.

5.1.1 Method

The Method token indicates the method to be performed on the
resource identified by the Request-URI. The method is case-sensitive.

Method = "OPTIONS" ; Section 9.2

"GET" ; Section 9.3

"HEAD" ; Section 9.4

"POST" ; Section 9.5

"PUT" ; Section 9.6

"DELETE" ; Section 9.7

"TRACE" ; Section 9.8

"CONNECT" ; Section 9.9

extension-method extension-method = token

The list of methods allowed by a resource can be specified in an
Allow header field (section 14.7). The return code of the response
always notifies the client whether a method is currently allowed on a
resource, since the set of allowed methods can change dynamically. 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; however, if the above methods are implemented,
they MUST be implemented with the same semantics as those specified
in section 9.