Bug

`rawString([.castNilToNSNull: true])` (and `rawString()` generally) can produce invalid JSON for two reasons:

1. Dict keys are interpolated unescaped

In `Source/SwiftyJSON/SwiftyJSON.swift` (around line 600), dictionary keys are written as:

```swift
""(key)": ..."
```

A key containing a double-quote or newline (both valid Swift `String` values) produces output like:

```json
{"key with "quotes"": 1} // invalid JSON
{"key with
newline": 1} // invalid JSON
```

2. String values escape only `\` and `"`, not control characters

Around line 610 (dict) and 641 (array), string values escape `\` → `\\` and `"` → `\"` but leave control characters (`\n`, `\t`, `\r`, U+0000–U+001F) unescaped. Per RFC 8259 §7, all control characters must be escaped in JSON strings.

```swift
let str = JSON(["msg": "line1\nline2"])
print(str.rawString()!)
// {"msg": "line1
// line2"} ← bare newline, invalid JSON — JSONSerialization.jsonObject rejects this
```

Impact

`JSON(obj: [String: Any]).rawString()` silently produces output that:

• Fails `JSONSerialization.jsonObject(with:)` round-trip
• Breaks any downstream JSON parser that enforces the spec

Existing partial fix

Closed-unmerged PR #1198 attempted to fix key-quoting but didn't cover control characters and was never merged. Open issue #746 is a different path (`.string` accessor). Neither addresses the control-character gap.

Fix

Route `rawString` output through `JSONSerialization` (which handles escaping correctly), or at minimum escape keys the same way values are escaped, and add `\n`→`\\n`, `\t`→`\\t`, `\r`→`\\r`, and `\u{XX}`→`\\u00XX` for control chars in both keys and values.