Added xml escaping as a function with replace function

[Joostluijben]

We found out the dotliquid does not support the xml escaping yet like in ruby . Therefore I implemented the function with only replace functions like shown here. I used the replace function because of the discussion found on this stackoverflow post

[codecov]

Codecov Report

Base: 88.26% // Head: 88.06% // Decreases project coverage by -0.19% ⚠️

Coverage data is based on head (3023703) compared to base (0d510d8).
Patch coverage: 0.00% of modified lines in pull request are covered.

❗ Current head 3023703 differs from pull request most recent head 61b6c40. Consider uploading reports for the commit 61b6c40 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
- Coverage   88.26%   88.06%   -0.20%     
==========================================
  Files          69       70       +1     
  Lines        2675     2681       +6     
  Branches      620      621       +1     
==========================================
  Hits         2361     2361              
- Misses        210      216       +6     
  Partials      104      104              

Impacted Files	Coverage Δ
src/DotLiquid/RubyFilters.cs	0.00% <0.00%> (ø)

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[microalps]

1. Please include tests to validate this new functionality. If this is in Ruby Liquid, needs to have a c# of all tests included from Ruby.
2. Makes sense to place in ExtendedFilters or ShopifyFilters and not in it's own RubyFilters

[microalps]

@Joostluijben We appreciate contributions, but please see my comments above. Also, codecov indicates that testing decreased, so please add whatever tests are needed to validate the functionality works as expected.

[microalps]

Kindly see above.