Fix CryptoUtils::assembleMessage() to not add bogus text/plain content-type

Found by failing unit tests mailcommon-filter-filteractiondecrypttest &
mailcommon-filter-filteractionencrypttest

src/util/cryptoutils.cpp

@@ -166,9 +166,17 @@ KMime::Message::Ptr CryptoUtils::assembleMessage(const KMime::Message::Ptr &orig
     out->setBody(const_cast<KMime::Content *>(newContent)->encodedBody());
     out->parse();
 
+    // remove default explicit content headers added by KMime::Content::parse()
+    QVector<KMime::Headers::Base *> headers = out->headers();
+    for (const auto hdr : std::as_const(headers)) {
+        if (isContentHeader(hdr)) {
+            out->removeHeader(hdr->type());
+        }
+    }
+
     // Copy over headers from the original message, except for CT, CTE and CD
     // headers, we want to preserve those from the new content
-    QVector<KMime::Headers::Base *> headers = orig->headers();
+    headers = orig->headers();
     for (const auto hdr : std::as_const(headers)) {
         if (isContentHeader(hdr)) {
             continue;