HTTP: fixed buffer_type inheritance in if blocks.

HTTP: fixed buffer_type inheritance in if blocks.

Previously, when js_body_filter was used inside an if block that
evaluated to true, the data parameter received Buffer type instead
of the expected String type. This happened because buffer_type field
in ngx_http_js_loc_conf_t was not properly initialized, causing the
configuration merge to fail when nginx created a new location context
for if blocks.

This fixes #999 issue on Github.