Refactor HttpServlet doOptions()

Now **doOptions()** has a duplicate code since it uses unnecessary flags variable to set **Allow Header**.

```
protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
Method[] methods = getAllDeclaredMethods(this.getClass());

boolean ALLOW_GET = false;
boolean ALLOW_HEAD = false;
boolean ALLOW_POST = false;
boolean ALLOW_PUT = false;
boolean ALLOW_DELETE = false;
boolean ALLOW_TRACE = true;
boolean ALLOW_OPTIONS = true;

for (int i = 0; i < methods.length; i++) {
String methodName = methods[i].getName();

if (methodName.equals("doGet")) {
ALLOW_GET = true;
ALLOW_HEAD = true;
} else if (methodName.equals("doPost")) {
ALLOW_POST = true;
} else if (methodName.equals("doPut")) {
ALLOW_PUT = true;
} else if (methodName.equals("doDelete")) {
ALLOW_DELETE = true;
}

}

// we know "allow" is not null as ALLOW_OPTIONS = true
// when this method is invoked
StringBuilder allow = new StringBuilder();
if (ALLOW_GET) {
allow.append(METHOD_GET);
}
if (ALLOW_HEAD) {
if (allow.length() > 0) {
allow.append(", ");
}
allow.append(METHOD_HEAD);
}
if (ALLOW_POST) {
if (allow.length() > 0) {
allow.append(", ");
}
allow.append(METHOD_POST);
}
if (ALLOW_PUT) {
if (allow.length() > 0) {
allow.append(", ");
}
allow.append(METHOD_PUT);
}
if (ALLOW_DELETE) {
if (allow.length() > 0) {
allow.append(", ");
}
allow.append(METHOD_DELETE);
}
if (ALLOW_TRACE) {
if (allow.length() > 0) {
allow.append(", ");
}
allow.append(METHOD_TRACE);
}
if (ALLOW_OPTIONS) {
if (allow.length() > 0) {
allow.append(", ");
}
allow.append(METHOD_OPTIONS);
}

resp.setHeader("Allow", allow.toString());
}
```

We can add allowed method to **Allow Header** while we **iterating over methods**.

Start by locating HttpServlet#doOptions() and read how it discovers supported methods and builds the Allow header. Refactor the iteration so allowed methods are added directly, while preserving the existing method order, separators, and TRACE/OPTIONS entries; done means the duplicate flags are gone and the header remains equivalent.

Written by the indexing model from the issue text.