Support r.URL.Host in host matcher

Previously, only r.Host from the incoming request was used to match
the expected mock host. This change supports r.URL.Host if r.Host
is not present.

Go http client docs specifies that both are valid:

```
For client requests Host optionally overrides the Host
header to send. If empty, the Request.Write method uses
the value of URL.Host. Host may contain an international
domain name.
```

mocks.go

@@ -398,6 +398,9 @@ var pathMatcher Matcher = func(r *http.Request, spec *MockRequest) error {
 
 var hostMatcher Matcher = func(r *http.Request, spec *MockRequest) error {
 	receivedHost := r.Host
+	if receivedHost == "" {
+		receivedHost = r.URL.Host
+	}
 	mockHost := spec.url.Host
 	if mockHost == "" {
 		return nil

mocks_test.go

@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"reflect"
 	"strings"
 	"testing"
@@ -54,19 +55,38 @@ func TestMocks_NewEmptyUnmatchedMockError_ExpectedErrorsString(t *testing.T) {
 }
 
 func TestMocks_HostMatcher(t *testing.T) {
-	tests := []struct {
-		requestUrl    string
+	tests := map[string]struct {
+		request       *http.Request
 		mockUrl       string
 		expectedError error
 	}{
-		{"http://test.com", "https://test.com", nil},
-		{"https://test.com", "https://testa.com", errors.New("received host test.com did not match mock host testa.com")},
-		{"https://test.com", "", nil},
+		"matching": {
+			request:       httptest.NewRequest(http.MethodGet, "http://test.com", nil),
+			mockUrl:       "https://test.com",
+			expectedError: nil,
+		},
+		"not matching": {
+			request:       httptest.NewRequest(http.MethodGet, "https://test.com", nil),
+			mockUrl:       "https://testa.com",
+			expectedError: errors.New("received host test.com did not match mock host testa.com"),
+		},
+		"no expected host": {
+			request:       httptest.NewRequest(http.MethodGet, "https://test.com", nil),
+			mockUrl:       "",
+			expectedError: nil,
+		},
+		"matching using URL host": {
+			request: &http.Request{URL: &url.URL{
+				Host: "test.com",
+				Path: "/",
+			}},
+			mockUrl:       "https://test.com",
+			expectedError: nil,
+		},
 	}
-	for _, test := range tests {
-		t.Run(fmt.Sprintf("%s %s", test.requestUrl, test.mockUrl), func(t *testing.T) {
-			req := httptest.NewRequest(http.MethodGet, test.requestUrl, nil)
-			matchError := hostMatcher(req, NewMock().Get(test.mockUrl))
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			matchError := hostMatcher(test.request, NewMock().Get(test.mockUrl))
 			assert.Equal(t, test.expectedError, matchError)
 		})
 	}