dev: add guard clause to AutoFixtureAdapter.
dev: implement the Fixture property.
refactor: change the constructor signature,

AutoFixtureAdapter(ISpecimenContext) ->
AutoFixtureAdapter(IFixture)
refactor: delete AutoFixtureAdapter.SpecimenContext.
dev: implement returning AutoFixtureAdapter itself.
Merge remote-tracking branch 'origin/testfixture-instance'
build(DEPLOYMENT): increment patch version,

to indicate that it is a fix rather than a new feature that
AutoFixtureAdapter returns itself for a request as type of
ITestFixture and AutoFixtureAdpater.