IMAPProtocol: Problem with searching emails when the subject contains umlauts

*Some Background*
We are searching in the IMAP-Server for specific Emails based on their Subject. We noticed in one of our tests that the search with umlauts (ü or ä) in the subject is not performant as it should be. It takes 30 Minuten to find 1 Email in the Inbox with > 6000 Emails.

We investigated the problem and found, that in this case IMAP-Server throws an Exception and the Library falls down to the default implementation and loads all Emails.

*Details*
We debugged the code and found the root-cause of the error.

Method `search` in the `IMAPProtocol` class https://github.com/eclipse-ee4j/angus-mail/blob/master/providers/imap/src/main/java/org/eclipse/angus/mail/imap/protocol/IMAPProtocol.java#L2494 has the following code:
```
// Check if the search "text" terms contain only ASCII chars,
// or if utf8 support has been enabled (in which case CHARSET
// is not allowed; see RFC 6855, section 3, last paragraph)
if (supportsUtf8() || SearchSequence.isAscii(term)) {
try {
return issueSearch(msgSequence, term, null);
} catch (IOException ioex) { /* will not happen */ }
}
```

Out IMAP-Server Supports UTF-8 and the code correctly calls `issueSearch` with no Charset. So far so good
The problem occurs in the `issueSearch` itself on line 2552 https://github.com/eclipse-ee4j/angus-mail/blob/master/providers/imap/src/main/java/org/eclipse/angus/mail/imap/protocol/IMAPProtocol.java#L2552

Here all `SearchTerm`s will be converted to the `Argument`
```
// Generate a search-sequence with the given charset
Argument args = getSearchSequence().generateSequence(term,
charset == null ? null :
MimeUtility.javaCharset(charset)
```
In our case the `charset` is NULL and then the subject from the SearchTerm will be converted as follows:
```
public Argument writeString(String s, String charset) throws UnsupportedEncodingException {
if (charset == null) {
this.writeString(s);
} else {
this.items.add(new AString(s.getBytes(charset)));
}

return this;
}
```
at the end `ASCIIUtility.getBytes(s)` will be called and it uses a default OS-Charset (on Windows it is not UTF-8) and at this point of time all umlaut have a wrong representation in the byte-array, which will be sent to the IMAP-Server.

We strongly believe that there should be a possibility to specify the Encoding for converting SearchTerms independently. Or maybe you can find more elegant solution.

Thanks in advance.

Start in providers/imap/src/main/java/org/eclipse/angus/mail/imap/protocol/IMAPProtocol.java at search and issueSearch, then trace SearchTerm generation into Argument.writeString and ASCIIUtility.getBytes. Reproduce a subject search containing ü or ä against an IMAP server with UTF-8 support; done means the server receives the correct representation and the search does not fall back to loading every message.

Written by the indexing model from the issue text.