Ensure the X509Certificate tag is properly prefixed

[casewalker]

Creating a signature using the default getKeyInfoContent function was missing prefixes on the X509Certificate tags, e.g.:

    ...
    <ds:KeyInfo>
        <ds:X509Data> 
            <X509Certificate>MII...</X509Certificate>
       </ds:X509Data>
    </ds:KeyInfo>
    ...

This PR should fix that. I modified an existing test to stop stubbing out the default getKeyInfoContent so that the existing behavior broke that test, then validated that the new behavior causes it to pass.

[codecov]

Codecov Report

Merging #377 (e1e19b4) into master (2aa2d13) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
+ Coverage   74.24%   74.35%   +0.11%     
==========================================
  Files           9        9              
  Lines         889      889              
  Branches      236      236              
==========================================
+ Hits          660      661       +1     
  Misses        135      135              
+ Partials       94       93       -1     

Files	Coverage Δ
src/signed-xml.ts	77.08% <100.00%> (+0.23%)	⬆️

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