Gzip: fixed setting of NGX_HTTP_GZIP_BUFFERED.

In r2411 setting of NGX_HTTP_GZIP_BUFFERED in c->buffered was moved from
ngx_http_gzip_filter_deflate_start() to ngx_http_gzip_filter_buffer() since
it was always called first.  But in r2543 the "postpone_gzipping" directive
was introduced, and if postponed gzipping is disabled (the default setting),
ngx_http_gzip_filter_buffer() is not called at all.

We must always set NGX_HTTP_GZIP_BUFFERED after the start of compression
since there is always a trailer that is buffered.

There are no known cases when it leads to any problem with current code.
But we already had troubles in upcoming SPDY implementation.

src/http/modules/ngx_http_gzip_filter_module.c

diff --git a/src/http/modules/ngx_http_gzip_filter_module.c b/src/http/modules/ngx_http_gzip_filter_module.c
index 2f39543..35dd07e 100644
--- a/src/http/modules/ngx_http_gzip_filter_module.c
+++ b/src/http/modules/ngx_http_gzip_filter_module.c
@@ -620,6 +620,8 @@
         return NGX_ERROR;
     }
 
+    r->connection->buffered |= NGX_HTTP_GZIP_BUFFERED;
+
     ctx->last_out = &ctx->out;
     ctx->crc32 = crc32(0L, Z_NULL, 0);
     ctx->flush = Z_NO_FLUSH;