Fix issue 866

Fix issue 866 by adding a const qualifier to what the F macro casts to.

Author: AdamCDunlap

hardware/arduino/cores/arduino/WString.h

@@ -35,7 +35,7 @@
 //     -std=c++0x
 
 class __FlashStringHelper;
-#define F(string_literal) (reinterpret_cast<__FlashStringHelper *>(PSTR(string_literal)))
+#define F(string_literal) (reinterpret_cast<const __FlashStringHelper *>(PSTR(string_literal)))
 
 // An inherited class for holding the result of a concatenation.  These
 // result objects are assumed to be writable by subsequent concatenations.