Preserve custome SCL, SDA and Frequency when Wire.begin() called

Changed how default parameters are interpreted.  Now calling `Wire.begin()` without parameters will reuse the current values instead of resetting them to the default values.  This change was prompted when it was found than custom pin assignments were being lost.  The standard Arduino library practice is to issue a `Wire.begin()` call inside each library that uses `Wire()`.  If a custom pin assignment was used, when these libraries were init'd, they caused a failure when they re-assigned the SDA and SCL pins.  Changing Frequency back to 100khz was not a fatal problem.

Author: stickbreaker

libraries/Wire/src/Wire.h

@@ -70,8 +70,10 @@ class TwoWire: public Stream
 public:
     TwoWire(uint8_t bus_num);
     ~TwoWire();
-    void begin(int sda=-1, int scl=-1, uint32_t frequency=100000);
-    void setClock(uint32_t);
+    void begin(int sda=-1, int scl=-1, uint32_t frequency=0); 
+       //defaults bus:0 sda=SDA, scl=SCL, frequency =100khz via variant pins_arduino.h
+       // bus:1 unspecified, emits Log_E()
+    void setClock(uint32_t); // change bus clock without initing hardware
     void beginTransmission(uint16_t);
     uint8_t endTransmission(bool);
 		uint8_t	requestFrom(uint16_t address, uint8_t size, bool sendStop);
@@ -88,7 +90,7 @@ class TwoWire: public Stream
     bool getDump(){return _dump;}
     void dumpInts();
     void dumpI2C(){i2cDumpI2c(i2c);}
-    size_t getClock();
+    size_t getClock(); // current bus clock rate in hz
     void setTimeOut(uint16_t timeOutMillis);
     uint16_t getTimeOut();
 //		
@@ -140,6 +142,7 @@ extern TwoWire Wire;
 
 
 /*
+V0.2.2 13APR2018 preserve custom SCL,SDA,Frequency when no parameters passed to begin()
 V0.2.1 15MAR2018 Hardware reset, Glitch prevention, adding destructor for second i2c testing
 */
 #endif