HttpClient crash inside {.exportc.} proc

HttpClient causes the program to crash if it is created or accessed inside of a proc with {.exportc.}

C: 
```
extern "C" test();
int main() {
    test()
}
```
Nim Ex 1:
```
proc test*() {.exportc.} =
    let client = newHttpClient() <-- crash
```
Nim Ex 2:
```
let client = newHttpClient()
proc test*() {.exportc.} =
    client.onProgressChanged = nil <-- crash
```
