=htc #16488 add missing assertions in tests

 - replaced pure `===` by `shouldEqual`
 - fixed the backport of spray/spray#927 in ae8c47acc170907ee9c8dfbcb946b400035f6877
 - fixed HttpHeaderParserSpec

@sirthias I disabled two tests in `HttpHeaderParserSpec` because the results have changed and I cannot say if that's an issue or correct behavior.