Fix EEPROM::begin missing underscore

Added size validation to the EEPROM begin, but forgot to set the class
variable with the new adjusted size.
Fixes #10

libraries/EEPROM/EEPROM.cpp

@@ -40,7 +40,7 @@ void EEPROMClass::begin(size_t size) {
         size = 4096;
     }
 
-    size = (size + 255) & (~255);  // Flash writes limited to 256 byte boundaries
+    _size = (size + 255) & (~255);  // Flash writes limited to 256 byte boundaries
 
     // In case begin() is called a 2nd+ time, don't reallocate if size is the same
     if (_data && size != _size) {