This might be easier to read with a two line helper method. Something like:
```ruby
def new_http_conn_error(url, parser, body)
  body_msg = parser.http_method == 'post' ? " with body '#{body}'" : ''
  "puffing-billy: Connection to #{url}#{body_msg} not cached and new http connections are disabled"
end
```
That's just my preference though.