Bug fixes and improvements

[Sebbo94BY]

• Issue [BUG] $mail->textPlain (and $mail->textHtml) has concatenated attachment's contents #390: Fixed, that attached HTML files got merged into the textHtml property
• Issue [Feature Request] Return original string instead of exception #397: convertStringEncoding() will now always return a string and never throw an exception
• Issue [BUG] Downloading attachments one by one and setting path and filename #398: Fixed issue, that files could not be manually (one by one) saved to disk using saveToDisk()
• Issue [BUG] Issue with decodeMimeStr() when value is only a whitespace #400: Fixed issue, that empty() returned a wrong result for strings with only whitespaces
• Updated some PhpDoc comment-blocks
• Improved code of DataPartInfo to be more human-readable
• Removed wrong if-condition in DataPartInfo::fetch()
• Fixed a few PhpCS warnings (eg. Expected type X, but Y given)
• Resorted $attachment property assignments based on IncomingMailAttachment properties order