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.
Each resulting symbol occupies one byte in the returned byte array.
Use encode to get the output in string form.
endIndex - the end (exclusive) of the subrange to encode, size of the source array by default.
Return a ByteArray with the resulting symbols.