Update string class to upstream version

CHANGELOG

 SAMD CORE
 
- * Fixed Serial.flush() blocking before any writes. Thanks @hangmoh
- * Added snprintf_P to avr/pgmspace.h stub. Thanks @jantje
+* Fixed Serial.flush() blocking before any writes. Thanks @hangmoh
+* Added snprintf_P to avr/pgmspace.h stub. Thanks @jantje
+* Another small fix to String iterators. Thanks @Ivan-Perez @Chris--A
 
 SAMD CORE 1.6.6 2016.05.19
 

cores/arduino/WString.h

@@ -159,10 +159,12 @@ public:
 	char& operator [] (unsigned int index);
 	void getBytes(unsigned char *buf, unsigned int bufsize, unsigned int index=0) const;
 	void toCharArray(char *buf, unsigned int bufsize, unsigned int index=0) const
-		{getBytes((unsigned char *)buf, bufsize, index);}
-	const char * c_str() const { return buffer; }
-	const char* begin() { return c_str(); }
-	const char* end() { return c_str() + length(); }
+		{ getBytes((unsigned char *)buf, bufsize, index); }
+	const char* c_str() const { return buffer; }
+	char* begin() { return buffer; }
+	char* end() { return buffer + length(); }
+	const char* begin() const { return c_str(); }
+	const char* end() const { return c_str() + length(); }
 
 	// search
 	int indexOf( char ch ) const;