Fallback to base64_decode if imap_base64 fails

Some mail clients and servers (like the ones developed in the northwestern 
United States) sometimes place encoded and non-encoded data in an email body
declared with "Content-Transfer-Encoding: base64". imap_base64 will refuse to
decode the body if it contains non base64 characters (like a period, colon,
apostrophe, etc). This patch uses base64_decode as a fallback which will make
a best effort to decode the data. In the case of our test mails, the decoded
data will contain garbage where the non-base64-encoded data was in the
original mail, but at least the message is preserved.