Fixed js_body_filter when data is not in memory.

Fixed js_body_filter when data is not in memory.

Previously, when upstream data was delivered from nginx cache
js_body_filter was not able to process it correctly. In particular,
it was treated as a chain of empty buffers.

The fix is to set r->filter_need_in_memory flag, which ensures
that ngx_http_core_module reads the data into memory before
js_body_filter sees it.

This fixes #992 issue on Github.