Support for Ruby 3.0

[jotolo]

When using this gem with ruby 3.0.3, I get the following error:

NoMethodError:
       undefined method `encode' for URI:Module

I checked the reason behind it and the method encode was removed in ruby 3.0

@nquocvuong prepared #6 to solve this. Can someone check this PR, please? Thanks in advance
Thanks @nquocvuong for the PR!

[michaelschmitz]

I am also working with a new version of Ruby and Rails at this point and running into the issue. It seems this is not merged yet. Can someone from the team do that or is sign_request the wrong gem now?