const correctness (#89)

make IPAddress.toString() const method, it doesn't modify anything
within the class.

cores/esp32/IPAddress.cpp

@@ -72,7 +72,7 @@ size_t IPAddress::printTo(Print& p) const
     return n;
 }
 
-String IPAddress::toString()
+String IPAddress::toString() const
 {
     char szRet[16];
     sprintf(szRet,"%u.%u.%u.%u", _address.bytes[0], _address.bytes[1], _address.bytes[2], _address.bytes[3]);
@@ -119,4 +119,4 @@ bool IPAddress::fromString(const char *address)
     }
     _address.bytes[3] = acc;
     return true;
-}
+}
\ No newline at end of file

cores/esp32/IPAddress.h

@@ -81,7 +81,7 @@ public:
     IPAddress& operator=(uint32_t address);
 
     virtual size_t printTo(Print& p) const;
-    String toString();
+    String toString() const;
 
     friend class EthernetClass;
     friend class UDP;