Fix some F-string support in WString

Author: wdoekes

WString.cpp

@@ -38,6 +38,13 @@ String::String(const String &value)
 	*this = value;
 }
 
+String::String(const __FlashStringHelper *str)
+{
+	init();
+	const char *cstr = reinterpret_cast<const char*>(str);
+	if (cstr) copy(cstr, strlen(cstr));
+}
+
 #if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
 String::String(String &&rval)
 {

WString.h

@@ -80,7 +80,9 @@ class String
 	// marked as invalid ("if (s)" will be false).
 	String & operator = (const String &rhs);
 	String & operator = (const char *cstr);
-	String & operator = (const __FlashStringHelper *str);
+	String & operator = (const __FlashStringHelper *str) {
+	    return operator=(reinterpret_cast<const char*>(str));
+	}
        #if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
 	String & operator = (String &&rval);
 	String & operator = (StringSumHelper &&rval);