refactor: replace rest-client with net/http

[jay7x]

Closes #153

[Joris29]

Nice PR but some things need to be updated like the spec test and i'm sure not all references to rest-client gem are removed, as i can see it's still in the gemspec file.

[jay7x]

I spent a while trying to understand what's wrong with the test (besides that multipart/form-data regex).. I'm still unsure if the request body is created properly.. may need another "a while" to figure it out

[ekohl]

Nice. I like relying on fewer external dependencies.

[Joris29]

For the spec tests it seems it might need some extra webmock config.
In the rest-client code they disable webmock twice
https://github.com/rest-client/rest-client/blob/2c72a2e77e2e87d25ff38feba0cf048d51bd5eca/spec/integration/httpbin_spec.rb#L7-L13
https://github.com/rest-client/rest-client/blob/2c72a2e77e2e87d25ff38feba0cf048d51bd5eca/spec/integration/request_spec.rb#L4-L10

[jay7x]

Haha.. found why this test was failing: bblimke/webmock#959..
Webmock doesn't support set_form(). So now I have to choose.. either to stop using webmock for this test, or reimplement set_form() internals here to fill body in a way webmock can deal with.

Here is the actual implementation (this method is called internally): https://github.com/ruby/net-http/blob/master/lib/net/http/generic_request.rb#L314

[jay7x]

So I reimplemented the unit test with RSpec mocks. Looks good enough from my point of view.

[jay7x]

FYI, net/http respects http_proxy variables by default

[jay7x]

Updated to reduce the diff even more. Will do all the improvements in another PR.