If the size of the source array or its subrange is not an integral multiple of 3,
the result is padded with
'=' to an integral multiple of 4 symbols.
destinationOffset - the starting index in the destination array to write symbols to, 0 by default.
endIndex - the end (exclusive) of the subrange to encode, size of the source array by default.
IndexOutOfBoundsException - when the resulting symbols don't fit into the destination array starting at the specified destinationOffset,
or when that index is out of the destination array indices range.
Return the number of symbols written into destination array.