Minor refactoring for better testability

[nij4t]

The following PR is aimed to provide a way of testing the library's components without the need of a remote connection. This way the SOAPClient can be created with a custom RoundTripper injected to http.Client which can be used during the unit tests.

// Defining custom round tripper
type RoundTripFunc func(req *http.Request) *http.Response

func (f RoundTripFunc) RoundTrip(req *http.Request) (*http.Response, error) {
	return f(req), nil
}

// NewTestClient returns *http.Client
// with Transport replaced to avoid making real calls
func NewTestClient(fn RoundTripFunc) *http.Client {
	return &http.Client{
		Transport: RoundTripFunc(fn),
	}
}

// Creating httpClient with mocked response
fakeclient := NewTestClient(func(req *http.Request) *http.Response {
	return &http.Response{
		StatusCode: 200,
		Body:       ioutil.NopCloser(bytes.NewStringBuffer("Fake document")),
		Header:     make(http.Header),
	}
})

// When sending requests via our client 
// we will get the same response no matter
// what our request object is
res, _ := fakeclient.Get("http://www.example.com")
println(string(res.Body))
// Output: Fake document

[tiaguinho]

Hi @nij4t can you fix the conflicts?