Added framework reference for net45 to system.net.http

Author: MIchaelMainer

src/Microsoft.Graph/Microsoft.Graph.csproj

@@ -53,6 +53,7 @@ July 2018 Release Summary (version 1.10.0)
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'net45' ">
     <Reference Include="System" />
+    <Reference Include="System.Net.Http" />
     <Reference Include="Microsoft.CSharp" />
     <PackageReference Include="Newtonsoft.Json" Version="[6.0.1,12)" />
   </ItemGroup>