KV Value Update handles Data wrongly

### Confirm this is a Python library issue and not an underlying Cloudflare API issue.

- [x] This is an issue with the Python library

### Describe the bug

When trying to set a value in the KV Store `metadata` needs to be provided, but gets merged into the `value` and no meta data gets set.

```
In [29]: response = client.kv.namespaces.values.update(key_name="test", value="MY VALUE", metadata="{}", **kv.params)

In [30]: print(client.kv.namespaces.values.get(key_name="test", **kv.params).text())
{"metadata":"{}","value":"MY VALUE"}

In [31]: print(client.kv.namespaces.metadata.get(key_name="test", **kv.params) is None)
True
```
The problem seems to be caused from the whole body of the request being used as the actual value. The behaviour seems to be correct when manually specifying the request's content tye as `multipart/form-data`:

```
In [38]: response = client.kv.namespaces.values.update(key_name="test", value="MY VALUE", metadata="{}", **kv.params, extra_headers={"Content-Type": "multipart/form-data"})

In [39]: print(client.kv.namespaces.values.get(key_name="test", **kv.params).text())
MY VALUE

In [40]: print(client.kv.namespaces.metadata.get(key_name="test", **kv.params))
{}
```
This should probably be the default behaviour as it shouldn't be necessary to explicitly override the content type. On the other hand `metadata` should not be required in which case the whole body could be set as `value`

### To Reproduce

1. Call `values.update()` with a `value` and `metadata`. Both are set as the actual value.
2. `metadata.get()` doesn't return any metadata.
3. Metadata is returned with the value upon calling `values.get()`.

### Code snippets

```Python

```

### OS

Every OS

### Python version

3.12

### Library version

v4.0.0

エントリポイント values.update() と metadata.get() から始め、value、metadata、Content-Type がどのように送信されるかを調べます。両方のフィールドを含むリクエストを再現し、その後、意図した metadata の動作に対するリグレッションテストを追加します。metadata が分離して保存され、values.get() が value の一部として返さなくなれば完了です。

索引モデルが issue の本文から書いたものです。