[ bug ] PG实例，表名纯大写或者大小写混合不能查询表结构

### 重现步骤

- SQL 查询 -> 在线查询
- 选择PG实例 -> 选择数据库 -> 选择模式
- 查看表结构 -> 选择一个包含大小写或者大写的表名
- 最下面查询结果出现error
```
relation "adminusers" does not exist
```

### 期待结果和实际结果
期待结果：
- PG表名包含大小写或者纯大写，可以正常查询表结构。

实际结果:
- 查询结果出现error

### 截图


### 错误日志

```
[2020-04-02 16:48:28,002][DummyThread-3:139900950932296][task_id:default][pgsql.py:164][WARNING]- PgSQL命令执行报错，语句：select
col.column_name,
col.data_type,
col.character_maximum_length,
col.numeric_precision,
col.numeric_scale,
col.is_nullable,
col.column_default,
des.description
from
information_schema.columns col left join pg_description des on
col.table_name::regclass = des.objoid
and col.ordinal_position = des.objsubid
where table_name = 'AdminUsers'
order by ordinal_position;， 错误信息：Traceback (most recent call last):
File "/opt/archery/sql/engines/pgsql.py", line 152, in query
cursor.execute(sql)
psycopg2.errors.UndefinedTable: relation "adminusers" does not exist
```

### 版本信息
应用版本/分支： 1.7.7

部署方式：Docker

Start in /opt/archery/sql/engines/pgsql.py around the query method and inspect the PostgreSQL schema query shown in the error log. Reproduce the issue with a mixed-case or uppercase table name, then verify that viewing its table structure completes without an UndefinedTable error.

Written by the indexing model from the issue text.