Describe the bug
I have prepared my custom package_wmarkow_index.json - I just needed to build with some latest GCC compiler. It contains two custom avr-1.8.2 platforms which uses two different compiler tools:

• avr-gcc-8.3.0-x64-mingw hosted as https://blog.zakkemble.net/download/avr-gcc-8.3.0-x64-mingw.zip
• avr-gcc-9.1.0-x64-mingw hosted as https://blog.zakkemble.net/download/avr-gcc-9.1.0-x64-mingw.zip

When I want to use one of these platforms in Sloeber, then compiler is not downloaded correctly, because the downloader doesn't follow the HTTP redirections to reach the destination.

To Reproduce
Take a look at the GIF:

Expected behavior
The tools should be downloaded.
As a workaround I downloaded them manually and copied into arduinoPlugin\downloads directory.

Screenshots
Take a look at the attached GIF above.

Desktop (please complete the following information):

• OS: Windows 7
• Browser Firefox 71.0
• Sloeber V4.3.1_win64.2018-10-10_08-21-58