bson: fix memory leaks when appending documents or arrays

[elbywan]

Issue

Calling append_document or append_array was leaking memory asLibBSON.bson_append_document_begin or LibBSON.bson_append_array_begin are not supposed to be called with an already allocated bson handler.

Fix

Replaced bson_append_document_begin + bson_append_document_end combo with a single bson_append_document call. Same fix for arrays.