Submits trailer fields HEADERS against the stream *stream_id*.

The *nva* is an array of name/value pair :type:`nghttp2_nv` with
*nvlen* elements.  The application must not include pseudo-header
fields (headers whose names starts with ":") in *nva*.

This function creates copies of all name/value pairs in *nva*.  It
also lower-cases all names in *nva*.  The order of elements in
*nva* is preserved.  For header fields with
:enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
:enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
header field name and value are not copied respectively.  With
:enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
is responsible to pass header field name in lowercase.  The
application should maintain the references to them until
:type:`nghttp2_on_frame_send_callback` or
:type:`nghttp2_on_frame_not_send_callback` is called.

For server, trailer fields must follow response HEADERS or response
DATA without END_STREAM flat set.  The library does not enforce
this requirement, and applications should do this for themselves.
If `nghttp2_submit_trailer()` is called before any response HEADERS
submission (usually by `nghttp2_submit_response2()`), the content
of *nva* will be sent as response headers, which will result in
error.

This function has the same effect with `nghttp2_submit_headers()`,
with flags = :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM` and both
pri_spec and stream_user_data to NULL.

To submit trailer fields after `nghttp2_submit_response2()` is
called, the application has to specify
:type:`nghttp2_data_provider2` to `nghttp2_submit_response2()`.
Inside of :type:`nghttp2_data_source_read_callback2`, when setting
:enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF`, also set
:enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_END_STREAM`.  After
that, the application can send trailer fields using
`nghttp2_submit_trailer()`.  `nghttp2_submit_trailer()` can be used
inside :type:`nghttp2_data_source_read_callback2`.

This function returns 0 if it succeeds and *stream_id* is -1.
Otherwise, this function returns 0 if it succeeds, or one of the
following negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory.
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
    The *stream_id* is 0.