Client class assumes we receive a XML as a response.

[imanoleguskiza]

On the send method of the EC\Poetry\Client we assume the response from the server is going to be an XML. This is a problem because if we don't (we receive an error string for example) we will trigger a Parsing Exception, which is not correct.

A solution would be to check if the response is a properly formatter XML and if it isn't simply return the string or throw an exception.