WebBrowserComponent: Fix adding custom HTTP headers on Linux

[paulgration]

Adds the missing functionality of being able to send additional custom HTTP headers with WebBrowserComponent requests on Linux as described in the documentation.

See forum post: Juce WebBrowserComponent not sending additional HTTP headers

Example usage:

std::unique_ptr<juce::StringArray> headers = std::make_unique<juce::StringArray> (
    "Hello: World",
    "Foo: Bar"
);

webView->goToURL ("http://localhost:5000", headers.get());