Previously, OrderedRequestContextFilter was ordered such that it
ran after Spring Security's Filter. This meant that the request
context was unavailable to any Filters in Spring Security's Filter
chain. Specifically, this caused a failure when using @EnableOAuth2Sso
as OAuth2ClientAuthenticationProcessingFilter, which is added to
Spring Security's Filter chain would be unable to use the
request-scoped bean upon which it depends.

This commit updates the order of OrderedRequestContextFilter so that
the request context is set up before Spring Security's Filter runs.
The tests for SampleGitHubApplication have been updated to use
TestRestTemplate rather than MockMvc. This is necessary as the latter,
via ServletTestExecutionListener, automatically populates the request
context holder, masking the fact that the request context filter was
setting it up too late.

Closes gh-4270