change charset to lowercase

I found a bug.
in MySQLdb below code is OK.

```python
   g_db=pymysql.connect(user=env["user"],
                         passwd=env["passwd"],
                         db=env["db"],
                         host=env["host"],
                         use_unicode=True,
                         charset='UTF8')
```

but in PyMySQL, it causes error because it has "utf8" as lowcases