get_system_metadata() with Format=STANDARD-XML does not capture `SystemID`, `SystemDescription` or `TimeZoneOffset` attributes

[ivanhm]

<?xml version="1.0"?>
<RETS ReplyCode="0" ReplyText="SUCCESS" >
    <METADATA>
        <METADATA-SYSTEM Version="19.10.36933" Date="2019-10-25T15:33:19.7Z">
            <SYSTEM SystemID="ABCMLS" SystemDescription="ABC MLS" TimeZoneOffset="Z">
                <COMMENTS></COMMENTS>
            </SYSTEM>
        </METADATA-SYSTEM>
    </METADATA>
</RETS>

A call to rets_client.get_system_metadata() with Format=STANDARD-XML and rets 1.7.2 does not return SystemID, SystemDescription or TimeZoneOffset attributes due to a small bug in the code.
I will create a PR with the fix.

In RETS 1.7.2 specification (https://www.rapattoni.com/rapdocs/support/mls/retsdocs/RETS_1_7_2.pdf), the SystemID, SystemDescriptions and TimeZoneOffset are the attributes of the <SYSTEM> tag as opposed to actual separate tags.

Here is the RETS 1.5 specification (https://www.reso.org/download/rets-1-5-specification-dtd-documents/)