previously we expected to find option elements only within select element and if
that was not the case we throw an error. This made it impossible to include datalist
element with nested option elements in the template.
Closes#1165
Previously only when ngOptions was used, we correctly handled situations
when model was set to an unknown value. With this change, we'll add/remove
extra unknown option or reuse an existing empty option (option with value
set to "") when model is undefined.
