Restore request body after reading in sign method

Fixes awslabs#225

The problem is the the `sign` method in ProxyClient reads the `req.Body` and that results
in a closed reader.

When the request is actually send to the transport later, the body cannot be read and results
in a body length of 0 which does not match the passed req.ContentLength.

We tried to write tests for this, but the tests are not fully matching the real world because:
* they use io.NopCloser
* they use a HTTP mock client that doesn't actually read the body to send to the proxy (makes sense,
its a mock, but it's different than the actual implementation)

handler/proxy_client.go

@@ -59,6 +59,7 @@ func (p *ProxyClient) sign(req *http.Request, service *endpoints.ResolvedEndpoin
 		}
 
 		body = bytes.NewReader(b)
+		req.Body = io.NopCloser(bytes.NewReader(b))
 	}
 
 	// S3 service should not have any escaping applied.