This is a limitation HttpUrlConnection. You can use Apache or OkHttp, which support PATCHas an alternative client. This can be done explicitly in the builder:
RestAdapter restAdapter = new RestAdapter.Builder()
.setEndpoint(..)
.setClient(new OkClient())
.build()
, X-HTTP-Method-Override . POST, PATCH . , .
, Retrofit UrlConnectionClient , HTTP. , , , .