Restricted keepalive_disable safari to OS X only.

The problem doesn't affect non-Apple systems for sure, and many pretend
to be Safari now.

Prodded by Piotr Sikora.

src/http/ngx_http_request.c

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index a8515f3..441517e 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -1493,7 +1493,9 @@
         } else if (ngx_strstrn(user_agent, "Chrome/", 7 - 1)) {
             r->headers_in.chrome = 1;
 
-        } else if (ngx_strstrn(user_agent, "Safari/", 7 - 1)) {
+        } else if (ngx_strstrn(user_agent, "Safari/", 7 - 1)
+                   && ngx_strstrn(user_agent, "Mac OS X", 8 - 1))
+        {
             r->headers_in.safari = 1;
 
         } else if (ngx_strstrn(user_agent, "Konqueror", 9 - 1)) {