Insufficient initialization in Serial1.begin()

Board: Arduino Due,
Board library: Arduino SAM Boards 1.6.12

'Hello' does not appear in the output of Serial1(TX1) in the following program.
Uncomment 'PA11 DISABLE' line, then it works.
```
void setup(){
Serial1.begin(115200);
Serial1.end();
pinMode(18,OUTPUT); //TX1 override
//*(uint32_t*)0x400e0e04=0x0800;//PA11 DISABLE (PIO Controller PIO Disable Register (PA) )
Serial1.begin(115200);
Serial1.println("Hello.");
}
void loop(){}
```
I don't think this is a usage that is often used, but I think that the pin configuration should be done every time.

regards,

Start by reproducing the Arduino Due sketch and trace the Serial1.begin() and Serial1.end() pin-configuration behavior, including the manual PA11 disable register write. Done means calling begin() again restores TX1 output after end() without that manual register change.

Written by the indexing model from the issue text.