Gerard Gunnewijk

HttpClient should only have 1 reusable object that exists for the lifetime of the application. See issue #62 This pull requests changed the usage pattern of the HttpClient

In the current implementation of the [`UrlChecker`](https://github.com/dotnet-architecture/HealthChecks/blob/dev/src/Microsoft.Extensions.HealthChecks/Internal/UrlChecker.cs) for every check a new HttpClient is created. This can cause problems. On [MSDN](https://msdn.microsoft.com/en-us/library/system.net.http.httpclient(v=vs.110).aspx#Remarks) it is stated under the remarks section: > HttpClient...

I was working on my own PDF library with strings & encodings. And I remembered my previous PR here. The current way Unicode & BigEndianUnicode strings are written is incorrect....