problem of copying a String with nul byte in the middle

After uploading my sketch (using Arduino 1.6.8 and Arduino Uno),
```arduino
void loop()
{

String str1 = "ACDEFG";
String str2 = "HIP";

String str3 = str1 + '\0' + str2;

Serial.println(str1);
Serial.println(str1.length());
Serial.println(str2);
Serial.println(str2.length());
Serial.println(str3);
Serial.println(str3.length());
Serial.println();
delay(10000);
}
```

I’m expecting to get as a result something like this:

ACDEFG
6
HIP
3
ACDEFG0HIP
10

But the result is:

ACDEFG
6
HIP
3
ACDEFG `ah
10

I.e. if I understand the problem correctly the String object keeps the right length of str3, but while copying the ‘\0’ it decides the string is over, but it’s not. So in str3 I get right symbols before ‘\0’ and some garbage after.

Start with the Arduino sketch's loop() example and reproduce it on an Arduino Uno using Arduino 1.6.8. Trace String concatenation and copying when str1, '\0', and str2 form str3. Done means the embedded nul byte is handled consistently, str3 retains the reported length, and its output matches the expected result without garbage.

Written by the indexing model from the issue text.