Improve request body io-alike objects handling #469

As of #468 pipes will cause failure:

reader, writer = IO.pipe
writer.puts "abc"
writer.close

HTTP.post(url, :body => reader)

That is because reader is an IO that supports rewind, but it's illegal on pipes:

Errno::ESPIPE: Illegal seek

So, I think we should become more strict about what is allowed to be used as request body.

@httprb/core comments and thoughts are welcomed and appreciated!