Add SharpDevelop user agent in NuGet feed requests.
Use NuGet-Operation install HTTP header when adding packages from dialog.
Use NuGet-Operation update HTTP header when updating packages from dialog.
Use NuGet-Operation install HTTP header when manage packages button used.
Use NuGet-Operation install HTTP header when using Install-Package cmdlet.
Use NuGet-Operation update HTTP header when using Update All button.
Refactor package management unit tests.
Use NuGet-Operation update HTTP header when using Update-Package cmdlet.