String class not behaving as expected with Print class on M0+

Initializing a String object with a float and then printing it fails on the M0+ platform.

Try this:

```
void setup() {
Serial.begin(9600);
}

void loop() {
  float foo = 3.14159269;
  String fee = String(foo, 2);
  Serial.print(foo, 5);
  Serial.print(" ");
  Serial.println(fee);
}
```

This should produce

3.14159 3.14

Instead it only produces 
3.14159
