[PATCH] Support DELETE verb in http_open/3

As requested by Wouter Beek, this patch adds support for HTTP 1.1 verb
DELETE.  Its implementation is very similar to GET and HEAD.

I actually needed this for a project I'm working.  Without this patch,
I got this:

    ?- http_open('http://127.0.0.1:3000/foo', Stream, [method(delete)]),
    |    close(Stream).
    ERROR: Domain error: `method' expected, found `delete'

With this patch, I get:

    Stream = <stream>(0x7fb71212e070).

and the correct request is made to the server.
---
 http_open.pl | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git http_open.pl http_open.pl
index 4014ecd..1d0a96c 100644
--- http_open.pl
+++ http_open.pl
@@ -54,7 +54,7 @@ two additional modules that acts as plugins:
 
     * library(http/http_header)
     Loading this library causes http_open/3 to support the =POST= method
-    in addition to =GET= and =HEAD=.
+    in addition to =GET=, =HEAD= and =DELETE=.
 
     * library(http/http_ssl_plugin)
     Loading this library causes http_open/3 HTTPS connections.  Relevant
@@ -106,7 +106,7 @@ resource. See also parse_time/2.
 		     [ authorization(compound),
 		       final_url(-atom),
 		       header(+atom, -atom),
-		       method(oneof([get,head,post])),
+		       method(oneof([delete,get,head,post])),
 		       size(-integer),
 		       status_code(-integer),
 		       timeout(number),
@@ -155,7 +155,8 @@ user_agent('SWI-Prolog').
 %	  AtomValue is unified to the empty atom ('').
 %
 %	  * method(+Method)
-%	  One of =get= (default) or =head=.   The  =head= message can be
+%	  One of =get= (default), =head= or =delete=.
+%	  The  =head= message can be
 %	  used in combination with  the   header(Name,  Value) option to
 %	  access information on the resource   without actually fetching
 %	  the resource itself.  The  returned   stream  must  be  closed
@@ -348,6 +349,7 @@ method(Options, MNAME) :-
 	;   domain_error(method, M)
 	).
 
+map_method(delete, 'DELETE').
 map_method(get,  'GET').
 map_method(head, 'HEAD').
 map_method(post, 'POST') :-
@@ -472,6 +474,7 @@ redirect_options(Options0, Options) :-
 	;   Options = Options1
 	).
 
+redirect_method(delete).
 redirect_method(get).
 redirect_method(head).
 
-- 
1.8.2.3