Edited Feature: HttpHeaders collections should benefit from IEnumerable covariance [1127]

Every HttpHeaders collections access from HttpRequestHeaders, HttpResponseHeaders, HttpContentHeaders that takes one of these type parameter:
MediaTypeWithQualityHeaderValue , NameValueWithParametersHeaderValue, TransferCodingWithQualityHeaderValue
should use their less specialized type parameter (without With_X_ in the type name) to benefit from IEnumerable<T> covariance. These collections construction or update should integrate default settings or type casting for Quality or Parameter but their concrete type should stay the more specialized one.