Fix idle level when initializing a inverted SoftwareSerial

Previously, when SoftwareSerial was initialized, it would always be set
to an idle level of HIGH, even when inverted logic was enabled. Once a
byte is transmitted, the idle level gets correctly set to LOW instead.
This commit makes sure that the idle level is correct directly after
initialization already.

This fixes arduino#1361.

Author: jenscski

libraries/SoftwareSerial/SoftwareSerial.cpp

@@ -355,7 +355,7 @@ SoftwareSerial::~SoftwareSerial()
 void SoftwareSerial::setTX(uint8_t tx)
 {
   pinMode(tx, OUTPUT);
-  digitalWrite(tx, HIGH);
+  digitalWrite(tx, _inverse_logic ? LOW : HIGH);
   _transmitBitMask = digitalPinToBitMask(tx);
   uint8_t port = digitalPinToPort(tx);
   _transmitPortRegister = portOutputRegister(port);