Enable CONNECT w/ response body tests with excon adapter

[technoweenie]

This is different than the httpclient error (#1). The response returns 200, but the body is empty. Looks like a consequence of how Go's HTTP server handles CONNECT requests.

  1) Faraday with HTTP server behaves like a connection making requests with excon: #CONNECT behaves like an idempotent request sends User-Agent request header
     Failure/Error: JSON.parse(response.body)

     JSON::ParserError:
       767: unexpected token at ''
     Shared Example Group: "any request expecting a response body" called from ./spec/support/request_examples.rb:51
     Shared Example Group: "an idempotent request" called from ./spec/support/request_examples.rb:19
     Shared Example Group: "a connection making requests" called from ./spec/insecure_spec.rb:5
     # ./spec/support/request_examples.rb:85:in `block (2 levels) in <top (required)>'
     # ./spec/support/request_examples.rb:97:in `block (3 levels) in <top (required)>'