Fixed string constructor overloading bug

(Filtered from arduino/Arduino@5bc28a1)

Author: majenkotech

core/WString.cpp

@@ -100,14 +100,14 @@ 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];

core/WString.h

@@ -68,8 +68,8 @@ class String
 	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=6);
-    explicit String(double, int decimalPlaces=6);
+    explicit String(float, unsigned char decimalPlaces=6);
+    explicit String(double, unsigned char decimalPlaces=6);
 	~String(void);
 
 	// memory management