ExclusiveCanonicalization: neighbouring comments are not all stripped when `WithComments: false`

When there are runs of multiple comments like  then the second one is not stripped. This doesn't happen if there is whitespace between the comments.

It seems like all comments should be stripped, but I checked just to be sure in case it's some weird quirk of the Canonical XML spec:

The second parameter of input to the XML canonicalization method is a boolean flag indicating whether or not comments should be included in the canonical form output by the XML canonicalization method. If a canonical form contains comments corresponding to the comment nodes in the input node-set, the result is called canonical XML with comments. Note that the XPath data model does not create comment nodes for comments appearing within the document type declaration (DTD). Implementations are REQUIRED to be capable of producing canonical XML excluding all comments that may have appeared in the input document or document subset. Support for canonical XML with comments is RECOMMENDED.

Here's a minimal project test case which reproduces the issue:

package main

import (
    "github.com/moov-io/signedxml"
    "github.com/stretchr/testify/assert"
    "github.com/stretchr/testify/require"
    "testing"
)

func TestExclusiveCanonicalization_WithNeighbouringComments(t *testing.T) {
    str := `
<a>
  <!-- comment0 --><!-- comment1 -->
</a>
`
    ec := signedxml.ExclusiveCanonicalization{WithComments: false}
    str, err := ec.Process(str, "")
    require.NoError(t, err)
    assert.Equal(t, `
<a>
</a>`, str)
}

Output:

=== RUN   TestExclusiveCanonicalization_WithNeighbouringComments
    main_test.go:19: 
            Error Trace:    main_test.go:19
            Error:          Not equal: 
                            expected: "\n<a>\n</a>"
                            actual  : "\n<a>\n  <!-- comment1 -->\n</a>"

                            Diff:
                            --- Expected
                            +++ Actual
                            @@ -2,2 +2,3 @@
                             <a>
                            +  <!-- comment1 -->
                             </a>
            Test:           TestExclusiveCanonicalization_WithNeighbouringComments
--- FAIL: TestExclusiveCanonicalization_WithNeighbouringComments (0.00s)

With a comment run like:

 Then the output will be:

So it seems like a comment immediately following another comment with no whitespace between them will reduce in comments being missed for stripping.

moov-io / signedxml

ExclusiveCanonicalization: neighbouring comments are not all stripped when `WithComments: false` #50