URL encoding for extended ASCII characters was producing the wrong re…

[benPearce1]

Fixes OctopusDeploy/Issues#3718
Added base unit tests around UrlTemplate

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[markryd]

I had a look at the history, and pre-netcore we were using Uri.HexEscape, which we then ported, so presumably this was always broken. EscapeDataString and HexEscape give different values for standard chars (HexEscape converts them to hex anyway) but I guess if it works with Octopus Server then we weren't relying on that ¯_(ツ)_/¯