Add articles_case to elision token filter (#3572)

Closes #3570

(cherry picked from commit 08025cd)

Author: russcam

src/Nest/Analysis/TokenFilters/ElisionTokenFilter.cs

@@ -13,29 +13,42 @@ public interface IElisionTokenFilter : ITokenFilter
 		/// </summary>
 		[JsonProperty("articles")]
 		IEnumerable<string> Articles { get; set; }
+
+		/// <summary>
+		/// Whether articles should be handled case-insensitively. Defaults to <c>false</c>.
+		/// </summary>
+		[JsonProperty("articles_case")]
+		bool? ArticlesCase { get; set; }
 	}
 
-	/// <inheritdoc />
+	/// <inheritdoc cref="IElisionTokenFilter"/>
 	public class ElisionTokenFilter : TokenFilterBase, IElisionTokenFilter
 	{
 		public ElisionTokenFilter() : base("elision") { }
 
 		/// <inheritdoc />
 		public IEnumerable<string> Articles { get; set; }
+
+		/// <inheritdoc />
+		public bool? ArticlesCase { get; set; }
 	}
 
-	/// <inheritdoc />
+	/// <inheritdoc cref="IElisionTokenFilter"/>
 	public class ElisionTokenFilterDescriptor
 		: TokenFilterDescriptorBase<ElisionTokenFilterDescriptor, IElisionTokenFilter>, IElisionTokenFilter
 	{
 		protected override string Type => "elision";
 
 		IEnumerable<string> IElisionTokenFilter.Articles { get; set; }
+		bool? IElisionTokenFilter.ArticlesCase { get; set; }
 
-		/// <inheritdoc />
+		/// <inheritdoc cref="IElisionTokenFilter.Articles"/>
 		public ElisionTokenFilterDescriptor Articles(IEnumerable<string> articles) => Assign(a => a.Articles = articles);
 
-		/// <inheritdoc />
+		/// <inheritdoc cref="IElisionTokenFilter.Articles"/>
 		public ElisionTokenFilterDescriptor Articles(params string[] articles) => Assign(a => a.Articles = articles);
+
+		/// <inheritdoc cref="IElisionTokenFilter.ArticlesCase"/>
+		public ElisionTokenFilterDescriptor ArticlesCase(bool? articlesCase = true) => Assign(a => a.ArticlesCase = articlesCase);
 	}
 }

src/Tests/Tests/Analysis/TokenFilters/TokenFilterUsageTests.cs

@@ -51,7 +51,8 @@ public class TokenFilterUsageTests : PromiseUsageTestBase<IIndexSettings, IndexS
 					elision = new
 					{
 						type = "elision",
-						articles = new[] { "a", "b", "c" }
+						articles = new[] { "a", "b", "c" },
+						articles_case = false
 					},
 					hunspell = new
 					{
@@ -329,6 +330,7 @@ public class TokenFilterUsageTests : PromiseUsageTestBase<IIndexSettings, IndexS
 					)
 					.Elision("elision", t => t
 						.Articles("a", "b", "c")
+						.ArticlesCase(false)
 					)
 #pragma warning disable 618 // Use of HunSpell IgnoreCase
 					.Hunspell("hunspell", t => t
@@ -506,7 +508,7 @@ public class TokenFilterUsageTests : PromiseUsageTestBase<IIndexSettings, IndexS
 							}
 						},
 						{ "etf", new EdgeNGramTokenFilter { MaxGram = 2, MinGram = 1 } },
-						{ "elision", new ElisionTokenFilter { Articles = new[] { "a", "b", "c" } } },
+						{ "elision", new ElisionTokenFilter { Articles = new[] { "a", "b", "c" }, ArticlesCase = false } },
 						{
 							"hunspell", new HunspellTokenFilter
 							{