Make the Common Name optional in CSRs

This change doesn't set it by default when adding domains, and adds a
method to explicitly set it if desired.

acme4j-client/src/main/java/org/shredzone/acme4j/util/CSRBuilder.java

@@ -78,11 +78,7 @@ public class CSRBuilder {
      *            Domain name to add
      */
     public void addDomain(String domain) {
-        var ace = toAce(requireNonNull(domain));
+        namelist.add(toAce(requireNonNull(domain)));
-        if (namelist.isEmpty()) {
-            namebuilder.addRDN(BCStyle.CN, ace);
-        }
-        namelist.add(ace);
     }
 
     /**
@@ -222,6 +218,15 @@ public class CSRBuilder {
         namebuilder.addRDN(oid, requireNonNull(value, "attribute value must not be null"));
     }
 
+    /**
+     * Sets the common name
+     * <p>
+     * Note that it is at the discretion of the ACME server to accept this parameter.
+     */
+    public void setCommonName(String cn) {
+        namebuilder.addRDN(BCStyle.CN, toAce(requireNonNull(cn)));
+    }
+
     /**
      * Sets the organization.
      * <p>