Fix truncated strings in function

Closes #468

Author: jberkel

Sources/SQLite/Core/Connection.swift

@@ -616,7 +616,7 @@ public final class Connection {
             } else if let result = result as? Int64 {
                 sqlite3_result_int64(context, result)
             } else if let result = result as? String {
-                sqlite3_result_text(context, result, Int32(result.count), SQLITE_TRANSIENT)
+                sqlite3_result_text(context, result, Int32(result.lengthOfBytes(using: .utf8)), SQLITE_TRANSIENT)
             } else if result == nil {
                 sqlite3_result_null(context)
             } else {

Tests/SQLiteTests/CustomFunctionsTests.swift

@@ -135,3 +135,12 @@ class CustomFunctionWithTwoArgsTests: SQLiteTestCase {
         XCTAssertEqual("ab", result)
     }
 }
+
+class CustomFunctionTruncation: SQLiteTestCase {
+    // https://github.com/stephencelis/SQLite.swift/issues/468
+    func testStringTruncation() {
+        _ = try! db.createFunction("customLower") { (value: String) in value.lowercased() }
+        let result = try! db.prepare("SELECT customLower(?)").scalar("TÖL-AA 12") as? String
+        XCTAssertEqual("töl-aa 12", result)
+    }
+}