Add language variant filtering support to the LanguageParameter

[petrsvihlik]

Motivation

Currently, the LanguageParameter does not support filtering based on language variants.

var response = await DeliveryClient.GetItemsAsync<Article>(new LanguageParameter("en-US"));

It would be nice if it was possible to ignore language fallbacks as described in the documentation:

This means that the LanguageParameter needs to be enriched with filtering based on system.language on top of the currently present language.

Design guidelines

Let's create two new constructors:

// This one will cause the system.language to be set instead of language
public LanguageParameter(string language, bool ignoreFallbacks = false)

// This one will set both language and system.language
public LanguageParameter(string language, string languageVariant = null)

The GetQueryStringParameter() method will need to be adjusted to return one or two query parameters in a single string.

• Don't forget to add unit tests