Bump System.Text.Encodings.Web (#607)

Bumps [System.Text.Encodings.Web](https://github.com/dotnet/corefx) from 4.5.0 to 4.5.1.
- [Release notes](https://github.com/dotnet/corefx/releases)
- [Commits](https://github.com/dotnet/corefx/commits)

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examples/WireMock.Net.StandAlone.Net461/packages.config

@@ -76,7 +76,7 @@
   <package id="System.Security.Cryptography.Primitives" version="4.3.0" targetFramework="net461" />
   <package id="System.Security.Cryptography.X509Certificates" version="4.3.0" targetFramework="net461" />
   <package id="System.Security.Principal.Windows" version="4.5.1" targetFramework="net461" />
-  <package id="System.Text.Encodings.Web" version="4.5.0" targetFramework="net461" />
+  <package id="System.Text.Encodings.Web" version="4.5.1" targetFramework="net461" />
   <package id="System.Threading.Tasks.Extensions" version="4.5.1" targetFramework="net461" />
   <package id="XPath2" version="1.1.0" targetFramework="net461" />
 </packages>