The `Cursor.fetch_dataframe` method doesn't respect case-sensitivity

## Driver version
2.1.3

## Redshift version
PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.2 20041017 (Red Hat 3.4.2-6.fc3), Redshift 1.0.76169

## Client Operating System
macos

## Python version
3.12.6

## Problem description

The `Cursor.fetch_dataframe` method [always lowercases](https://github.com/aws/amazon-redshift-python-driver/blob/64cbd54ef6a5e71d98ce193630d09267cc154379/redshift_connector/cursor.py#L526) column names, irrespective of the [case-sensitivity configuration value](https://docs.aws.amazon.com/redshift/latest/dg/r_enable_case_sensitive_identifier.html). This behavior is unexpected, because the resulting `DataFrame`'s columns may be treated as case-insensitive, even when that flag is set to `true`.

One example where this can be problematic is demonstrated below:

```python
from redshift_connector import connect

conn = connect()
cursor = conn.cursor()

cursor.execute('SET enable_case_sensitive_identifier TO true')
cursor.execute('WITH t AS (SELECT 1 AS "C", 2 AS "c") SELECT * FROM t')

# cursor.fetch_dataframe()
# c c
# 0 1 2

# cursor.fetch_dataframe().to_dict()
# :1: UserWarning: DataFrame columns are not unique, some columns will be omitted.
# {'c': {0: 2}}
```

## Possible solutions

I see that there is an [open PR](https://github.com/aws/amazon-redshift-python-driver/pull/232) related to this issue, but I don't think it solves it. I believe that the correct way to solve this is to get rid of the `lower()` call in line 526 altogether. That would mean that the columns produced by the driver reflect those returned by Redshift, hence respecting the case-sensitivity configuration value (see image below). I plan to open a PR with this fix soon.

----

P.S.: I see that the line of interest was introduced in the [first commit](https://github.com/aws/amazon-redshift-python-driver/blame/64cbd54ef6a5e71d98ce193630d09267cc154379/redshift_connector/cursor.py#L526) of this repo (!) and hasn't changed since. My hunch is that this was most likely an oversight, since Redshift's [documentation](http://web.archive.org/web/20200804231541/https://docs.aws.amazon.com/redshift/latest/dg/welcome.html) at the date of that commit had no mention of the `enable_case_sensitive_identifier` flag, so the driver must've not been updated to take it into account after it was introduced.

Bắt đầu trong redshift_connector/cursor.py tại phần triển khai fetch_dataframe quanh dòng 526, nơi tên cột được chuyển thành chữ thường. Tái hiện vấn đề với enable_case_sensitive_identifier được đặt thành true và truy vấn trả về cả "C" và "c". Được xem là hoàn tất khi DataFrame kết quả giữ nguyên tên các cột do Redshift trả về, bao gồm cả sự khác biệt giữa chữ hoa và chữ thường.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.