[Bug]: HTTPBody `asString` property always returns `nil`

### Platform Version

iOS 17.2

### SDK Version

1.8.3

### Xcode Version

Xcode 15.2

### Steps To Reproduce

Let's suppose we do have a struct representing a form:
```swift
struct MyForm: Encodable {
let numbers: [Int]
}
```
if we create a `HTTPBody` object by passing an instance of `MyForm`:
```swift
let form = try! HTTPBody.form(object: MyForm(numbers: [1, 2, 3]))
```
calling `form.asString` always returns `nil`.

### Expected Behavior

`form.asString` should returns a String representing the `MyForm` instance we have passed.

### Actual Incorrect Behavior

`form.asString` always returns `nil`.

Locate the HTTPBody type and inspect how the asString property is produced, then reproduce the issue with the MyForm example on iOS 17.2 and SDK 1.8.3. Done means form.asString returns a String representing the encoded MyForm value instead of nil, with regression coverage if the project has tests for HTTPBody.

Written by the indexing model from the issue text.