Describe the bug
Encountered unexpected token: "IF" "IF" when drop table if exists table1

To Reproduce
Steps to reproduce the behavior:

1. Example SQL
   drop table if exists table1

2. Parsing this SQL using JSqlParser with this statements
   String sqlTemp = "drop table if exists table1 ";
   CCJSqlParserManager parserManager = new CCJSqlParserManager();
   Statement parser = parserManager.parse(new StringReader(sqlTemp));

3. Exception
   Encountered unexpected token: "IF" "IF" when drop table if exists table1

Expected behavior
parser.toString() == "drop table if exists table1", not exception

System

• Database mysql 5.6
• JDK1.8
• JSqlParser 0.9.2

### resolve it, modify the code :
final public Drop Drop() throws ParseException {
Drop drop = new Drop();
Token tk = null;
List dropArgs = new ArrayList();
jj_consume_token(K_DROP);
switch ((jj_ntk == -1) ? jj_ntk_f() : jj_ntk) {
case S_IDENTIFIER: {
tk = jj_consume_token(S_IDENTIFIER);
break;
}
case K_TABLE: {
tk = jj_consume_token(K_TABLE);
break;
}
case K_INDEX: {
tk = jj_consume_token(K_INDEX);
break;
}
default:
jj_la1[257] = jj_gen;
jj_consume_token(-1);
throw new ParseException();
}
drop.setType(tk.image);

    // TODO : 需要判断 IF EXISTS
    switch ((jj_ntk == -1) ? jj_ntk_f() : jj_ntk) {
        case K_IF: {
            jj_consume_token(K_IF);
            jj_consume_token(K_EXISTS);
            drop.setIfExists(true);
            break;
        }
    }

    // TODO : 可能是带引号的字符串
    switch ((jj_ntk == -1) ? jj_ntk_f() : jj_ntk) {
        case S_IDENTIFIER: {
            tk = jj_consume_token(S_IDENTIFIER);
            break;
        }
        case S_QUOTED_IDENTIFIER: {
            tk = jj_consume_token(S_QUOTED_IDENTIFIER);
            break;
        }
    }

    // tk = jj_consume_token(S_IDENTIFIER);
    drop.setName(tk.image);
    label_55:
    while (true) {
        switch ((jj_ntk == -1) ? jj_ntk_f() : jj_ntk) {
            case S_IDENTIFIER: {
                ;
                break;
            }
            default:
                jj_la1[258] = jj_gen;
                break label_55;
        }
        tk = jj_consume_token(S_IDENTIFIER);
        dropArgs.add(tk.image);
    }
    if (dropArgs.size() > 0)
        drop.setParameters(dropArgs);
    {
        if ("" != null) return drop;
    }
    throw new Error("Missing return statement in function");
}

and Drop.java
public class Drop implements Statement {

private String type;
private String name;
private List<String> parameters;

// TODO : 添加ifExists
private boolean ifExists = false;

@Override
public void accept(StatementVisitor statementVisitor) {
    statementVisitor.visit(this);
}

public String getName() {
    return name;
}

public List<String> getParameters() {
    return parameters;
}

public String getType() {
    return type;
}

public void setName(String string) {
    name = string;
}

public void setParameters(List<String> list) {
    parameters = list;
}

public void setType(String string) {
    type = string;
}

@Override
public String toString() {
    // TODO : 加入IF EXISTS判断
    String sql = "DROP " + type;
    if (ifExists) {
        sql = sql + " IF EXISTS ";
    }
    sql = sql + " " + name;

    if (parameters != null && parameters.size() > 0) {
        sql += " " + PlainSelect.getStringList(parameters);
    }

    return sql;
}

public boolean isIfExists() {
    return ifExists;
}

public void setIfExists(boolean ifExists) {
    this.ifExists = ifExists;
}