Fix for bug in MailMessage reported on forum

(http://pocoproject.org/forum/viewtopic.php?f=12&t=6106&sid=9283682f915ea148d86813ea140cd3c7).
MailMessage was not handling encoding when no Content-Transfer-Encodign
header was not present for MultiPart messages, resulting on an
Exception. His fix checks if the header is present before getting is
value, and defaults to 7bit encoding as in
http://www.w3.org/Protocols/rfc1341/5_Content-Transfer-Encoding.html

Net/testsuite/src/MailMessageTest.h

@@ -53,9 +53,11 @@ public:
 	void testWriteMultiPart();
 	void testReadWriteMultiPart();
 	void testReadWriteMultiPartStore();
+	void testReadDefaultTransferEncoding();
 	void testReadQP();
 	void testRead8Bit();
 	void testReadMultiPart();
+	void testReadMultiPartDefaultTransferEncoding();
 	void testEncodeWord();
 
 	void setUp();