The failure message of XCTAssertNil shouldn't output the value as optional

This fix makes it match Darwin XCTest again.

Author: briancroom

Sources/XCTest/XCTAssert.swift

@@ -282,11 +282,10 @@ public func XCTAssertLessThanOrEqual<T : Comparable>(@autoclosure expression1: (
 
 public func XCTAssertNil(@autoclosure expression: () throws -> Any?, @autoclosure _ message: () -> String = "", file: StaticString = __FILE__, line: UInt = __LINE__) {
     _XCTEvaluateAssertion(.Nil, message: message, file: file, line: line) {
-        let value = try expression()
-        if value == nil {
-            return .Success
-        } else {
+        if let value = try expression() {
             return .ExpectedFailure("\"\(value)\"")
+        } else {
+            return .Success
         }
     }
 }