Correctly dispose HttpClient in its example

[svick]

Fixes dotnet/dotnet-api-docs#1045.

Also switches from explicitly calling Dispose to using, because I think that's a better practice.

In the process of testing this change, I used the following csproj:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net45;netcoreapp1.0</TargetFrameworks>
    <LangVersion>7.1</LangVersion>
  </PropertyGroup>

  <ItemGroup Condition="'$(TargetFramework)' == 'net45'">
    <Reference Include="System.Net.Http" />
  </ItemGroup>

</Project>

Would it be useful if I included it in this PR?

[richlander]

Separately, should we add a usage of HttpClientFactory?

[svick]

@richlander

1. Isn't that specific to ASP.NET Core? I mean, it's technically usable outside of it, but is using it actually a recommended practice in, say, WPF or Xamarin?
2. Wouldn't that require lots of dependency injection-related code? I think that doesn't belong here.

Though adding a link to a document about it might make sense.

[BillWagner]

Thanks for making these changes @svick

I've approved this change, and I'll now.

Thanks again for all your contributions.

[BillWagner]

@svick:

Would it be useful if I included it (the csproj) in this PR?

What we've been doing as time permits is migrating the snippets into the dotnet/samples repository, and making them part of a buildable project. That does require 2 PRs: one in samples to create the new sample, and a separate one in dotnet/docs to refer to the new sample and remove the old code. I didn't want to hold up a good PR on that process though.