UARTClass::write function loses the first character when it is called many times in series.

Series like this:
```
Serial.print("r=");
Serial.print(25);
Serial.print(",");
Serial.print(1);
Serial.print(";");
```
`
Output: r=25,1;r=25,1;r=25,1;r=5,1;r=25,1;r=25,1;
`

Temporary fix (in UARTClass.cpp):
```
size_t UARTClass::write( const uint8_t uc_data )
{
int nextWrite = (_tx_buffer->_iHead + 1);
if(nextWrite >= SERIAL_BUFFER_SIZE) nextWrite = 0;
while (_tx_buffer->_iTail == nextWrite)
; // Spin locks if we're about to overwrite the buffer. This continues once the data is sent

_tx_buffer->_aucBuffer[_tx_buffer->_iHead] = uc_data;
_tx_buffer->_iHead = nextWrite;

// Make sure TX interrupt is enabled
_pUart->UART_IER = UART_IER_TXRDY;
return 1;
}
```

Start in UARTClass.cpp at UARTClass::write(const uint8_t uc_data) and reproduce the issue with the repeated Serial.print series shown here. Compare the current output with the temporary implementation in the issue, then verify that repeated writes no longer lose the first character.

Written by the indexing model from the issue text.