http2: Ensure that http2_handle_stream_close is called

Previously, when HTTP/2 is enabled and used, and stream has content
length known, Curl_read was not called when there was no bytes left to
read. Because of this, we could not make sure that
http2_handle_stream_close was called for every stream. Since we use
http2_handle_stream_close to emit trailer fields, they were
effectively ignored. This commit changes the code so that Curl_read is
called even if no bytes left to read, to ensure that
http2_handle_stream_close is called for every stream.

Discussed in https://github.com/bagder/curl/pull/564