Add test for npn protocol in custom ssl.
user
```ruby
require 'spec_helper'

describe NetHttp2::Client do

  describe "npm protocols in SSL" do

    subject { client.instance_variable_get(:@ssl_context) }

    context "when no custom SSL context is passed in" do
      let(:client) { NetHttp2::Client.new("http://localhost") }

      it "specifies the DRAFT protocol" do
        expect(subject.npn_protocols).to eq ['h2']
      end
    end
    context "when a custom SSL context is passed in" do
      let(:ssl_context) { OpenSSL::SSL::SSLContext.new }
      let(:client) { NetHttp2::Client.new("http://localhost", ssl_context: ssl_context) }

      it "specifies the DRAFT protocol" do
        expect(subject.npn_protocols).to eq ['h2']
        expect(ssl_context.npn_protocols).to eq ['h2']
      end
    end
  end
end
```
assistant