MatchYAML compares only the first document of a multi-document YAML stream #933

This is another bug found by our automated process of writing hegel tests in hegel-zoo, filed as asked in #925 and, as before, reviewed by hand first.

MatchYAML reports a match when the actual YAML is a stream whose first document equals the expected: a: 1 matches a: 1\n---\nb: 2, and two streams that differ only in their second document match each other, while the same difference in the first document is caught:

package main

import (
	"fmt"

	"github.com/onsi/gomega"
)

func main() {
	ok, err := gomega.MatchYAML("a: 1").Match("a: 1\n---\nb: 2")
	fmt.Println("a: 1  vs  a: 1 --- b: 2:        ", ok, err)
	ok, err = gomega.MatchYAML("a: 1\n---\nb: 2").Match("a: 1\n---\nb: 3")
	fmt.Println("a: 1 --- b: 2  vs  a: 1 --- b: 3:", ok, err)
	ok, err = gomega.MatchYAML("a: 1\n---\nb: 2").Match("a: 2\n---\nb: 2")
	fmt.Println("a: 1 --- b: 2  vs  a: 2 --- b: 2:", ok, err)
}

Output:

a: 1  vs  a: 1 --- b: 2:         true <nil>
a: 1 --- b: 2  vs  a: 1 --- b: 3: true <nil>
a: 1 --- b: 2  vs  a: 2 --- b: 2: false <nil>

I expected the first two to be false, or for a stream with more than one document to be rejected the way invalid YAML is; as it stands the second and later documents are accepted and then have no effect on the result.

Tested on v1.43.1, which is also the current master (47f0f87).