DTR Off blocks USB serial receive

When I used USB serial in Arduino micro, I noticed that after opening USB serial connection with **DTR Off** everything goes OK. Then if I made **DTR On** it is still OK, but when I return to **DTR Off** receiveing stopped and recovers only if **DTR is ON**.
It does not correspond with ordinary USB to serial behavior, e.g. FT232 chip, which transmits and receves regardless **DTR** state.

Second issue: **RTS** state in Serial.rts() routine changed only when DTR state changes, i.e. **DTR** clocks **RTS**. It is also illegal behavior.

No file or test is named. Begin by locating the USB serial receive handling and the Serial.rts() routine in the AVR core, then reproduce the described DTR Off/On/Off sequence; done means receiving works with DTR Off and RTS changes independently of DTR.

Written by the indexing model from the issue text.