Add respecting Accept-Language header

[Azbesciak]

Will be applied as a fallback in case of missing lang query param, only if supported languages intersects languages in the header with header priority spec maintained.

[Azbesciak]

Any feedback?

[lonvia]

Sorry for the late review. So, I think this is useful in general. I have three comments about the implementation though:

1. It needs to be implemented for the ReverseRequestFactory as well.
2. What about the language strings like 'en-US', which are common in Accept-Language headers?
3. I find wrapping the LanguageChecker class a little bit overkill. I would just merge the new RequestLanguageResolver and LanguageChecker in one.

[otbutz]

2. What about the language strings like 'en-US', which are common in Accept-Language headers?

Works flawlessly using Locale.lookupTag()

Minimal working example:

List<String> supportedLangs = Arrays.asList("de", "en", "es", "fr");
String ranges = "fr-CH, fr-FR;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5";
List<Locale.LanguageRange> languageRanges = Locale.LanguageRange.parse(ranges);
String tag = Locale.lookupTag(languageRanges, supportedLangs);
System.out.println(tag); // returns "fr"

[lonvia]

Ah, so we only need a test for it. :)

[lonvia]

Closing in favour of #467.