[BUG] stats: support url-encoded forms

Bashkim Kasa reported that the stats admin page did not work when colons
were used in server or backend names. This was caused by url-encoding
resulting in ':' being sent as '%3A'. Now we systematically decode the
field names and values to fix this issue.
diff --git a/src/proto_http.c b/src/proto_http.c
index 1fc7c94..69232f9 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -2932,6 +2932,9 @@
 				*value++ = '\0';
 			}
 
+			if (!url_decode(key) || !url_decode(value))
+				break;
+
 			/* Now we can check the key to see what to do */
 			if (!backend && strcmp(key, "b") == 0) {
 				backend = value;