DTR Off blocks USB serial receive

Board: Arduino Micro(ATMEGA32U4)
Test Sketch:
```c++
void setup() {
Serial.begin(115200);
Serial1.begin(115200);
}
void loop() {
if (Serial.available()) {
Serial1.write(Serial.read());
}
if (Serial1.available()) {
Serial.write(Serial1.read());
}
}
```
To reproduce the issue make LoopBack connection RX-TX open Terminal Emulator(e.g. CoolTerm) and begin to send something. Switch DTR state ON/OFF.
If DTR OFF transmition stops.
It does not correspond with ordinary USB to serial behavior, e.g. FT232 chip, which transmits and receves regardless DTR state.

The same test runs on Teensy 2.0 with no such issue.

Reproduce the loopback test on an Arduino Micro using the supplied sketch and a terminal emulator, toggling DTR while sending data. Trace the USB serial receive path for the ATmega32U4 and compare its behavior with the reported Teensy 2.0 and FT232 cases. Done means receiving continues when DTR is off, without breaking normal serial behavior.

Written by the indexing model from the issue text.