Currently, if multiple keys are pushed to a builder without an associated value, all keys but the last one pushed are ignored.

Consider the following code snippet, which demonstrates this issue with a stream builder. A user might expect this code snippet to create the document {"foo": "bar", "baz": "quux"}, but instead it creates the document {"baz": "quux"}. To fix this issue, we should change the builders so that an exception is thrown when the "bar" key is attempted to be pushed.

builder::stream::document doc;
doc << "foo";
doc << "bar";
doc << "baz" << "quux";