Return https if r.TLS is not nil (#126)

`go-httpbin` was incorrectly returning `http` as the scheme for its URL
if you ran it with TLS specified via `HTTPS_CERT_FILE` and
`HTTPS_KEY_FILE`. Update it to return https as the scheme in this case
by checking if `r.TLS` is not nil

Before:

```
$ mkcert test
$ docker run -e HTTPS_CERT_FILE='/tmp/test.pem' -e HTTPS_KEY_FILE='/tmp/test-key.pem' -p 8080:8080 -v $(pwd):/tmp mccutchen/go-httpbin
$ curl -sk https://localhost:8080/get | jq -r .url
http://localhost:8080/get
```

After (TLS):

```
$ docker run -e HTTPS_CERT_FILE='/tmp/test.pem' -e HTTPS_KEY_FILE='/tmp/test-key.pem' -p 8080:8080 -v $(pwd):/tmp llimllib/go-httpbin
$ curl -sk https://localhost:8080/get | jq -r .url
https://localhost:8080/get
```

After (no TLS):

```
$ docker run -p 8080:8080 llimllib/go-httpbin
$ curl -sk http://localhost:8080/get | jq -r .url
http://localhost:8080/get
```

I got the idea for how to check the scheme from [this SO
post](https://stackoverflow.com/a/61446922/42559)

httpbin/helpers.go

@@ -59,6 +59,9 @@ func getURL(r *http.Request) *url.URL {
 	if scheme == "" && r.Header.Get("X-Forwarded-Ssl") == "on" {
 		scheme = "https"
 	}
+	if scheme == "" && r.TLS != nil {
+		scheme = "https"
+	}
 	if scheme == "" {
 		scheme = "http"
 	}

httpbin/helpers_test.go

@@ -1,9 +1,11 @@
 package httpbin
 
 import (
+	"crypto/tls"
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
 	"reflect"
 	"testing"
 	"time"
@@ -33,6 +35,82 @@ func assertError(t *testing.T, got, expected error) {
 	}
 }
 
+func mustParse(s string) *url.URL {
+	u, e := url.Parse(s)
+	if e != nil {
+		panic(e)
+	}
+	return u
+}
+
+func TestGetURL(t *testing.T) {
+	baseUrl, _ := url.Parse("http://example.com/something?foo=bar")
+	tests := []struct {
+		name     string
+		input    *http.Request
+		expected *url.URL
+	}{
+		{
+			"basic test",
+			&http.Request{
+				URL:    baseUrl,
+				Header: http.Header{},
+			},
+			mustParse("http://example.com/something?foo=bar"),
+		},
+		{
+			"if TLS is not nil, scheme is https",
+			&http.Request{
+				URL:    baseUrl,
+				TLS:    &tls.ConnectionState{},
+				Header: http.Header{},
+			},
+			mustParse("https://example.com/something?foo=bar"),
+		},
+		{
+			"if X-Forwarded-Proto is present, scheme is that value",
+			&http.Request{
+				URL:    baseUrl,
+				Header: http.Header{"X-Forwarded-Proto": {"https"}},
+			},
+			mustParse("https://example.com/something?foo=bar"),
+		},
+		{
+			"if X-Forwarded-Proto is present, scheme is that value (2)",
+			&http.Request{
+				URL:    baseUrl,
+				Header: http.Header{"X-Forwarded-Proto": {"bananas"}},
+			},
+			mustParse("bananas://example.com/something?foo=bar"),
+		},
+		{
+			"if X-Forwarded-Ssl is 'on', scheme is https",
+			&http.Request{
+				URL:    baseUrl,
+				Header: http.Header{"X-Forwarded-Ssl": {"on"}},
+			},
+			mustParse("https://example.com/something?foo=bar"),
+		},
+		{
+			"if request URL host is empty, host is request.host",
+			&http.Request{
+				URL:  mustParse("http:///just/a/path"),
+				Host: "zombo.com",
+			},
+			mustParse("http://zombo.com/just/a/path"),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			res := getURL(test.input)
+			if res.String() != test.expected.String() {
+				t.Fatalf("expected %s, got %s", test.expected, res)
+			}
+		})
+	}
+}
+
 func TestParseDuration(t *testing.T) {
 	okTests := []struct {
 		input    string