Fixed String class regression after f80c6c5f35cddcf4761a3c97feb8504425e9d27d

This should make explicit String-from-integer constructor working again:

   int a = 10;
   String(a, 4);

cores/arduino/WString.cpp

@@ -106,14 +106,14 @@ String::String(unsigned long value, unsigned char base)
 	*this = buf;
 }
 
-String::String(float value, int decimalPlaces)
+String::String(float value, unsigned char decimalPlaces)
 {
 	init();
 	char buf[33];
 	*this = dtostrf(value, (decimalPlaces + 2), decimalPlaces, buf);
 }
 
-String::String(double value, int decimalPlaces)
+String::String(double value, unsigned char decimalPlaces)
 {
 	init();
 	char buf[33];

cores/arduino/WString.h

@@ -69,8 +69,8 @@ public:
 	explicit String(unsigned int, unsigned char base=10);
 	explicit String(long, unsigned char base=10);
 	explicit String(unsigned long, unsigned char base=10);
-	explicit String(float, int decimalPlaces=2);
-	explicit String(double, int decimalPlaces=2);
+	explicit String(float, unsigned char decimalPlaces=2);
+	explicit String(double, unsigned char decimalPlaces=2);
 	~String(void);
 
 	// memory management
@@ -113,7 +113,7 @@ public:
 	String & operator += (const String &rhs)	{concat(rhs); return (*this);}
 	String & operator += (const char *cstr)		{concat(cstr); return (*this);}
 	String & operator += (char c)			{concat(c); return (*this);}
-	String & operator += (unsigned char num)	{concat(num); return (*this);}
+	String & operator += (unsigned char num)		{concat(num); return (*this);}
 	String & operator += (int num)			{concat(num); return (*this);}
 	String & operator += (unsigned int num)		{concat(num); return (*this);}
 	String & operator += (long num)			{concat(num); return (*this);}