Add the support for custom headers in both android and ios

[bugduino]

Fix needed for allowing the support of custom headers when doing

this.manager.makeRequest

example usage

return this.manager.makeRequest(provider, url, {
    method,
    headers: {
      'custom-header-1': 'foo',
      'custom-header-2': 'bar'
    },
    params
});

Joint work with @mrigo

replaces #220