SPDY: stop emitting multiple empty header values.

Previously, nginx would emit empty values in a header with multiple,
NULL-separated values.

This is forbidden by the SPDY specification, which requires headers to
have either a single (possibly empty) value or multiple, NULL-separated
non-empty values.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

src/http/ngx_http_spdy_filter_module.c

diff --git a/src/http/ngx_http_spdy_filter_module.c b/src/http/ngx_http_spdy_filter_module.c
index d1406a6..377e935 100644
--- a/src/http/ngx_http_spdy_filter_module.c
+++ b/src/http/ngx_http_spdy_filter_module.c
@@ -493,9 +493,13 @@
                 continue;
             }
 
-            *last++ = '\0';
+            if (h[j].value.len) {
+                if (last != p) {
+                    *last++ = '\0';
+                }
 
-            last = ngx_cpymem(last, h[j].value.data, h[j].value.len);
+                last = ngx_cpymem(last, h[j].value.data, h[j].value.len);
+            }
 
             h[j].hash = 2;
         }