Revert "Require Go 1.16"

This reverts commit 0e87d64ffc.

fs.go

@@ -1,3 +1,5 @@
+// +build go1.16
+
 package gemini
 
 import (

go.mod

@@ -1,5 +1,5 @@
 module git.sr.ht/~adnano/go-gemini
 
-go 1.16
+go 1.15
 
 require golang.org/x/net v0.0.0-20210119194325-5f4716e94777

response_test.go

@@ -2,6 +2,7 @@ package gemini
 
 import (
 	"io"
+	"io/ioutil"
 	"strings"
 	"testing"
 )
@@ -91,7 +92,7 @@ func TestReadWriteResponse(t *testing.T) {
 
 	for _, test := range tests {
 		t.Logf("%#v", test.Raw)
-		resp, err := ReadResponse(io.NopCloser(strings.NewReader(test.Raw)))
+		resp, err := ReadResponse(ioutil.NopCloser(strings.NewReader(test.Raw)))
 		if err != test.Err {
 			t.Errorf("expected err = %v, got %v", test.Err, err)
 		}
@@ -105,7 +106,7 @@ func TestReadWriteResponse(t *testing.T) {
 		if resp.Meta != test.Meta {
 			t.Errorf("expected meta = %s, got %s", test.Meta, resp.Meta)
 		}
-		b, _ := io.ReadAll(resp.Body)
+		b, _ := ioutil.ReadAll(resp.Body)
 		body := string(b)
 		if body != test.Body {
 			t.Errorf("expected body = %#v, got %#v", test.Body, body)