Current version (as it is in develop branch at commit 09c117f6db8bb2866229e1696c997a9677a1be30) is weak when a request without Content-Type is received. This is an example trace when that happens:
M:Tuesday 27 Aug 12:07:57 2013(512):contextBroker-/rest.cpp[193] connectionTreat: ------------------------ IN 4:0 -----------------------
T:Tuesday 27 Aug 12:07:57 2013(512):contextBroker-/rest.cpp[230] connectionTreat: HTTP method: POST, URL: /ngsi9/registerContext, version: HTTP/1.1
M:Tuesday 27 Aug 12:07:57 2013(512):contextBroker-/ConnectionInfo.h[56] ConnectionInfo: outFormat: XML
T:Tuesday 27 Aug 12:07:57 2013(512):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Cache-Control: no-cache
W:Tuesday 27 Aug 12:07:57 2013(512):contextBroker-/rest.cpp[68] httpHeaderGet: 'unsupported' HTTP header: 'Cache-Control', value 'no-cache'
T:Tuesday 27 Aug 12:07:57 2013(512):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Pragma: no-cache
W:Tuesday 27 Aug 12:07:57 2013(512):contextBroker-/rest.cpp[68] httpHeaderGet: 'unsupported' HTTP header: 'Pragma', value 'no-cache'
T:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Connection: keep-alive
W:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[68] httpHeaderGet: 'unsupported' HTTP header: 'Connection', value 'keep-alive'
T:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Cookie: csrftoken=tbxKrKA1PHi1zuwgd5uax4AZJoLogBpa
W:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[68] httpHeaderGet: 'unsupported' HTTP header: 'Cookie', value 'csrftoken=tbxKrKA1PHi1zuwAZJoLogBpa'
T:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Content-Type: text/plain; charset=UTF-8
T:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Content-Length: 690
T:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Accept-Encoding: gzip, deflate
W:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[68] httpHeaderGet: 'unsupported' HTTP header: 'Accept-Encoding', value 'gzip, deflate'
T:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Accept-Language: null
W:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[68] httpHeaderGet: 'unsupported' HTTP header: 'Accept-Language', value 'null'
T:Tuesday 27 Aug 12:07:57 2013(513):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Accept: application/xml
T:Tuesday 27 Aug 12:07:57 2013(514):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:23.o/20100101 Firefox/23.0
T:Tuesday 27 Aug 12:07:57 2013(514):contextBroker-/rest.cpp[59] httpHeaderGet: HTTP Header: Host: localhost:10026
M:Tuesday 27 Aug 12:07:57 2013(514):contextBroker-/rest.cpp[245] connectionTreat: contentLength == 690
E:Tuesday 27 Aug 12:07:57 2013(514):contextBroker-/rest.cpp[252] connectionTreat: MHD_create_post_processor FAILED
and nothing is responded after that FAILED.