Remove unnecessary header field in ErrorResponse (#1066)

ErrorResponse.Headers is causing a panic inside golang http package.

The following code can be easily crashed if GetObject API returns an error,
such as ErrSlowDown:

```
  reader, err := s3Client.GetObject("my-bucketname", "my-source-objectname", minio.GetObjectOptions{})
  if err != nil {
    log.Fatalln(err)
  }
  defer reader.Close()

  stat, err := reader.Stat()
  if err != nil {
    log.Fatalln(err)
  }

  n, err := s3Client.PutObject("my-bucketname", "my-target-objectname", reader, stat.Size, minio.PutObjectOptions{})
  if err != nil {
    log.Fatalln(err)
  }
```

The reason is that `reader` is passed to s3Client.PutObject therefore to golang http
via request.Body.  Since, reader.Read() can return an ErrorResponse, that error will
be tested by golang http, but since ErrorResponse is uncomparable, then it will cause
a crash with the following stackstrace:

```
  panic: runtime error: comparing uncomparable type minio.ErrorResponse

  goroutine 20 [running]:
  net/http.(*Request).write(0xc0002c2300, 0x761e60, 0xc000069f80, 0x0, 0x0, 0x0, 0x7628a0, 0xc000518780)
          /home/vadmeste/work/go/src/net/http/request.go:647 +0x74c
  net/http.(*persistConn).writeLoop(0xc0000a17a0)
          /home/vadmeste/work/go/src/net/http/transport.go:1888 +0x1b8
  created by net/http.(*Transport).dialConn
          /home/vadmeste/work/go/src/net/http/transport.go:1339 +0x966
  exit status 2
```

Hence, removing Headers since it is a map and it is uncomparable.

api-error-response.go

@@ -36,6 +36,8 @@ import (
 */
 
 // ErrorResponse - Is the typed error returned by all API operations.
+// ErrorResponse struct should be comparable since it is compared inside
+// golang http API (https://github.com/golang/go/issues/29768)
 type ErrorResponse struct {
 	XMLName    xml.Name `xml:"Error" json:"-"`
 	Code       string
@@ -51,9 +53,6 @@ type ErrorResponse struct {
 
 	// Underlying HTTP status code for the returned error
 	StatusCode int `xml:"-" json:"-"`
-
-	// Headers of the returned S3 XML error
-	Headers http.Header `xml:"-" json:"-"`
 }
 
 // ToErrorResponse - Returns parsed ErrorResponse struct from body and
@@ -177,9 +176,6 @@ func httpRespToErrorResponse(resp *http.Response, bucketName, objectName string)
 		errResp.Message = fmt.Sprintf("Region does not match, expecting region ‘%s’.", errResp.Region)
 	}
 
-	// Save headers returned in the API XML error
-	errResp.Headers = resp.Header
-
 	return errResp
 }
 

api-error-response_test.go

@@ -77,7 +77,6 @@ func TestHttpRespToErrorResponse(t *testing.T) {
 			RequestID:  resp.Header.Get("x-amz-request-id"),
 			HostID:     resp.Header.Get("x-amz-id-2"),
 			Region:     resp.Header.Get("x-amz-bucket-region"),
-			Headers:    resp.Header,
 		}
 		return errResp
 	}
@@ -292,3 +291,13 @@ func TestErrWithoutMessage(t *testing.T) {
 		t.Errorf("Expected \"Error response code InvalidArgument.\", got %s", errResp)
 	}
 }
+
+// Tests if ErrorResponse is comparable since it is compared
+// inside golang http code (https://github.com/golang/go/issues/29768)
+func TestErrorResponseComparable(t *testing.T) {
+	var e1 interface{} = ErrorResponse{}
+	var e2 interface{} = ErrorResponse{}
+	if e1 != e2 {
+		t.Fatalf("ErrorResponse should be comparable")
+	}
+}