Add header for http.head in LocalOrNetworkImageProvider

[Aoi-hosizora]

The http.head header should contain the headers parameter.

      int totalSize;
      http.head(url, headers: {
        'Accept': '*/*',
        'Accept-Encoding': '',
      }).then((data) {
        totalSize = int.tryParse(data.headers['content-length']);
      });

[Aoi-hosizora]

Done

var h = (headers ?? {})..['Accept-Encoding'] = '';
int totalSize;
http.head(url, headers: {
  'Accept': '*/*',
  ...h,
}).then((data) {
  totalSize = int.tryParse(data.headers['content-length']);
}).catchError((_) {});