dateEditor was serializing/deserializing wrong value. Also need to check for empty date value because moment will throw an error. `singleSelect` and `multipleSelectEditor` were serializing the actual value (e.g number type) when it should be serializing to a string and `applyValue` should be deserializing from string to actual type (e.g. number).

closes #56

not sure why i thought I should always make the serialized value a string. Probabaly because I was chaning the `dateEditor` at the same time. Anyway, the serailied value from the `valueName` property should always be a simple type.

this will allow the excel cell copy to work when serializing this.$input.value()