How to mock Net::HTTP::Post?

If you don't care about the exact instance, you could use the an_instance_of method:

http.should_receive(:request).with(an_instance_of(Net::HTTP::Post), foo: 1)
.and_return 202

Here it is with new syntax:

before do
  http = double
  allow(Net::HTTP).to receive(:start).and_yield http
  allow(http).to \
    receive(:request).with(an_instance_of(Net::HTTP::Get))
      .and_return(Net::HTTPResponse)
end

And then in example:

it "http" do
  allow(Net::HTTPResponse).to receive(:body)
    .and_return('the actual body of response')
  # here execute request
end

Very helpful if you need test external api library.