Don't validate RDAP nameserver names using validateDomainName

The nameserver may be external, in which case its TLD will not appear in our
list of valid TLDs, and the search will be rejected erroneously.

Tests for letter case canonicalizations also added at reviewer's suggestion.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171985702

java/google/registry/flows/domain/DomainFlowUtils.java

@@ -169,7 +169,7 @@ public class DomainFlowUtils {
    *
    * @see #validateDomainNameWithIdnTables(InternetDomainName)
    */
-  static InternetDomainName validateDomainName(String name)
+  public static InternetDomainName validateDomainName(String name)
       throws EppException {
     if (!ALLOWED_CHARS.matchesAllOf(name)) {
       throw new BadDomainNameCharacterException();