Ensure that the URL alphabet for base64 is used with the urlencode
functions (urlencode, urlencode_buffer, urlencode_temp and
urlencode_new) are used. Add a new test.
Fix the base64 decoding. If there's an 'A' character in the encoded
text, the base64 decode function returns an INVALID_PADDING error. The
reason lies in the way Base64Decoder.init tries to find a suitable
invalid character. Fix this by defining the invalid character as 0xff
(which is already the case for a decoding without padding).
This error has not been caught by the test harness, because no test
contains an 'A' character in the the encoded text yet. Add a new test.
