WiserAssertions.getMimeMessageBody: accept that content may be a String

Spring's SimpleMailMessage doesn't wrap it's email body in a mime content
type as it is just a 'simple' mail message.

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>

src/main/java/net/kemitix/wiser/assertions/WiserAssertions.java

@@ -181,6 +181,9 @@ public final class WiserAssertions {
     private String getMimeMessageBody(final WiserMessage message)
             throws IOException, MessagingException {
         Object content = getMimeMessage(message).getContent();
+        if (content instanceof String) {
+            return (String) content;
+        }
         if (content instanceof MimeMessage) {
             return content.toString();
         }