Accessing `allHTTPHeaderFields` makes a side effect on `URLRequest` equatability

Originator:ncreated
Number:rdar://FB8814193 Date Originated:Oct 20, 2020
Status:Open Resolved:
Product:Xcode Product Version:12.0 (12A7209)
Classification: Reproducible:Always
 
In Foundation framework, when using `URLRequest`, it seems that retrieving `allHTTPHeaderFields` makes a side effect visible in `Equatable` comparison. The following snippet run on iOS14 (Simulator or device) can be used to demonstrate it:
```
import Foundation

let url = URL(string: "https://api.com/foo")!

var request1 = URLRequest(url: url)
var request2 = URLRequest(url: url)

print(request1 == request2)

request1.setValue("Foo", forHTTPHeaderField: "Bar")
request2.setValue("Foo", forHTTPHeaderField: "Bar")

print(request1 == request2)

_ = request1.allHTTPHeaderFields

print(request1 == request2)
```

When starting the app, I’d expect following output:
```
true
true
true
```

Instead, I get this one:
```
true
true
false
```

Note: when running this snippet in macOS Playground, the output is as expected.

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