(bugfix) fix vt tag underscore replacement

[thomasht86]

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Closing #951.
Now, we can do:

 vt("max_matches", "2", replace_underscores=False)

Full test:

class TestUnderscoreAttributes(unittest.TestCase):
    def setUp(self):
        self.xml_schema = """<services version="1.0">
    <container id="colpalidemo_container" version="1.0">
        <search></search>
        <document-api></document-api>
        <document-processing></document-processing>
        <clients>
            <client id="mtls" permissions="read,write">
                <certificate file="security/clients.pem" />
            </client>
            <client id="token_write" permissions="read,write">
                <token id="colpalidemo_write" />
            </client>
            <client id="token_read" permissions="read">
                <token id="colpalidemo_read" />
            </client>
        </clients>
        <config name="container.qr-searchers">
            <tag>
                <bold>
                    <open>&lt;strong&gt;</open>
                    <close>&lt;/strong&gt;</close>
                </bold>
                <separator>...</separator>
            </tag>
        </config>
    </container>
    <content id="colpalidemo_content" version="1.0">
        <redundancy>1</redundancy>
        <documents>
            <document type="pdf_page" mode="index"></document>
        </documents>
        <nodes>
            <node distribution-key="0" hostalias="node1"></node>
        </nodes>
        <config name="vespa.config.search.summary.juniperrc">
            <max_matches>2</max_matches>
            <length>1000</length>
            <surround_max>500</surround_max>
            <min_length>300</min_length>
        </config>
    </content>
</services>
"""

    def test_valid_config_from_string(self):
        self.assertTrue(validate_services(self.xml_schema))

    def test_generate_schema(self):
        generated = services(
            container(
                search(),
                document_api(),
                document_processing(),
                clients(
                    client(
                        certificate(file="security/clients.pem"),
                        id="mtls",
                        permissions="read,write",
                    ),
                    client(
                        token(id="colpalidemo_write"),
                        id="token_write",
                        permissions="read,write",
                    ),
                    client(
                        token(id="colpalidemo_read"),
                        id="token_read",
                        permissions="read",
                    ),
                ),
                config(
                    vt("tag")(
                        vt("bold")(
                            vt("open", "<strong>"),
                            vt("close", "</strong>"),
                        ),
                        vt("separator", "..."),
                    ),
                    name="container.qr-searchers",
                ),
                id="colpalidemo_container",
                version="1.0",
            ),
            content(
                redundancy("1"),
                documents(document(type="pdf_page", mode="index")),
                nodes(node(distribution_key="0", hostalias="node1")),
                config(
                    vt("max_matches", "2", replace_underscores=False),
                    vt("length", "1000"),
                    vt("surround_max", "500", replace_underscores=False),
                    vt("min_length", "300", replace_underscores=False),
                    name="vespa.config.search.summary.juniperrc",
                ),
                id="colpalidemo_content",
                version="1.0",
            ),
            version="1.0",
        )
        generated_xml = generated.to_xml()
        # Validate against relaxng
        print(self.xml_schema)
        print(generated_xml)
        self.assertTrue(validate_services(str(generated_xml)))
        self.assertTrue(compare_xml(self.xml_schema, str(generated_xml)))