Adjust Swift_Mime_ContentEncoder_Base64ContentEncoder read() to non-triplet aligned number to make test fail.

Previously read(8190) was called, presumably because 8190 is the largest triplet-aligned number below 8k (8192).

However, read() doesn't guarantee to return all bytes requested, so this was a terrible way to triplet-align the streams.
Fix bug when read() does not return a buffer of length 3n which caused the base64 stream to be truncated.