Escape Guid value

[amoskvin]

This is a fix for invalid xml generated if guid is a url with unescaped xml characters:

    <item>
      <link>https://example.com/?foo=7&amp;bar=11</link>
      <guid>https://example.com/?foo=7&bar=11</guid>
    </item>

[codecov[bot]]

Codecov Report

Merging #98 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #98   +/-   ##
=======================================
  Coverage   79.52%   79.52%           
=======================================
  Files          24       24           
  Lines        1724     1724           
=======================================
  Hits         1371     1371           
  Misses        353      353           

Impacted Files	Coverage Δ
tests/read.rs	95.30% <ø> (ø)
src/guid.rs	92.30% <100.00%> (ø)

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[andy128k]

LGTM