Adding append

Sources/RedisNIO/Commands/StringCommands.swift

@@ -89,6 +89,20 @@ extension RedisClient {
             .map { return $0 == 1 }
     }
 
+    /// Append a value to the end of an existing entry
+    /// - Note: If the key does not exist it is created and set as an empty string, so APPEND will be similar to SET in this special case.
+    ///
+    /// [https://redis.io/commands/append](https://redis.io/commands/append)
+    /// - Parameters:
+    ///     - key: The key to use to uniquely identify this value.
+    ///     - value: The value to append the key to.
+    /// - Returns: Integer with the new length of the value
+    @inlinable
+    public func append(_ key: String, to value: RESPValueConvertible) -> EventLoopFuture<Int> {
+        return send(command: "APPEND", with: [key, value])
+            .convertFromRESPValue()
+    }
+    
     @usableFromInline
     func _mset(
         command: String,

Tests/RedisNIOTests/Commands/StringCommandsTests.swift

@@ -53,10 +53,21 @@ final class StringCommandsTests: XCTestCase {
         XCTAssertEqual(try connection.mget(["empty", #function]).wait().count, 2)
     }
 
-    func test_set() {
+    func test_set() throws {
         XCTAssertNoThrow(try connection.set(#function, to: "value").wait())
+        let val = try connection.get(#function).wait()
+        XCTAssertEqual(val, "value")
     }
-
+    
+    func test_append() throws {
+        let result = "value appended"
+        XCTAssertNoThrow(try connection.append(#function, to: "value").wait())
+        let length = try connection.append(#function, to: " appended").wait()
+        XCTAssertEqual(length, result.count)
+        let val = try connection.get(#function).wait()
+        XCTAssertEqual(val, result)
+    }
+    
     func test_mset() throws {
         let data = [
             "first": 1,
@@ -137,6 +148,7 @@ final class StringCommandsTests: XCTestCase {
         ("test_get", test_get),
         ("test_mget", test_mget),
         ("test_set", test_set),
+        ("test_append", test_append),
         ("test_mset", test_mset),
         ("test_msetnx", test_msetnx),
         ("test_increment", test_increment),