Add option to close tags in body when serializing

[tioback]

Hi,

Currently, when an object is serialized, there are body elements that don't get closed, such as:

	<SIGNONMSGSRSV1>
		<SONRS>
			<STATUS>
				<CODE>0
				<SEVERITY>INFO
			</STATUS>
			<DTSERVER>20210330
			<LANGUAGE>POR
			<FI>
				<ORG>Some Bank Out There
				<FID>0123
			</FI>
		</SONRS>
	</SIGNONMSGSRSV1>

It would be interesting if there was an option to close all tags, like:

const options: { closeTags: true }
ofx.serialize(header, body, options);

Which in turn would result in the following content:

	<SIGNONMSGSRSV1>
		<SONRS>
			<STATUS>
				<CODE>0</CODE>
				<SEVERITY>INFO</SEVERITY>
			</STATUS>
			<DTSERVER>20210330</DTSERVER>
			<LANGUAGE>POR</LANGUAGE>
			<FI>
				<ORG>Some Bank Out There</ORG>
				<FID>0123</FID>
			</FI>
		</SONRS>
	</SIGNONMSGSRSV1>

I'll work on a PR later on today.