Fix UnkeyedContainer crash for collections with nil (#200)

Sources/Leaf/LeafEncoder.swift

@@ -189,7 +189,9 @@ private final class UnkeyedContainer: UnkeyedEncodingContainer, _Container {
         } else {
             let encoder = _Encoder(codingPath: codingPath)
             try value.encode(to: encoder)
-            self.array.append(encoder.container!.data!)
+            if let safeData = encoder.container!.data {
+                self.array.append(safeData)
+            }
         }
     }
 

Tests/LeafTests/LeafTests.swift

@@ -1,6 +1,7 @@
 import Leaf
 import LeafKit
 import XCTVapor
+import Foundation
 
 class LeafTests: XCTestCase {
     func testApplication() throws {
@@ -258,6 +259,40 @@ class LeafTests: XCTestCase {
             XCTAssertEqual(res.status, .internalServerError)
         }
     }
+
+    // Test for GH Issue #197
+    func testNoFatalErrorWhenAttemptingToUseArrayWithNil() throws {
+        var test = TestFiles()
+        test.files["/foo.leaf"] = """
+        #(value)
+        """
+
+        let app = Application(.testing)
+        defer { app.shutdown() }
+        app.views.use(.leaf)
+        app.leaf.sources = .singleSource(test)
+
+        struct ArrayWithNils: Content {
+            let value:[UUID?]
+        }
+
+        let id1 = UUID.init()
+        let id2 = UUID.init()
+
+
+        app.get("noCrash") { req -> EventLoopFuture<View> in
+            let context = ArrayWithNils(value: [id1,nil,id2, nil])
+            return req.view.render("foo", context)
+        }
+
+        try app.test(.GET, "noCrash") { res in
+            // Expected result .ok
+            XCTAssertEqual(res.status, .ok)
+
+            // Rendered result should match to all non-nil values
+            XCTAssertEqual(res.body.string, "[\"\(id1)\", \"\(id2)\"]")
+        }
+    }
 }
 
 /// Helper `LeafFiles` struct providing an in-memory thread-safe map of "file names" to "file data"