public static FieldUpdate createAssign(Field field, FieldValue newValue) {
        return new FieldUpdate(field, ValueUpdate.createAssign(newValue));
    }
Creates a new field update, with one encapsulated value update,
that assigns a new value, completely overwriting the previous value. Note that it is possible to pass
newValue=null to this method to remove the value completely.

@param field    the Field to assign a new value to
@param newValue the value to assign
@return a FieldUpdate specifying the assignment
@throws IllegalArgumentException if the runtime type of newValue does not match the type required by field
@see com.yahoo.document.update.ValueUpdate#createAssign(FieldValue)
document/src/main/java/com/yahoo/document/update/FieldUpdate.java