Fix FBSDKLoginButton nonce property attributes (#1616)

Summary:
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## Pull Request Details

The attributes for the `nonce` property of the `FBSDKLoginButton` are incorrect and lead to build errors.

In scenarios where the [`objc-property-assign-on-object-type`](https://clang.llvm.org/docs/DiagnosticsReference.html#wobjc-property-assign-on-object-type) flag is enabled and warnings are treated as errors, the build fails with: `FBSDKLoginKit.framework/Headers/FBSDKLoginButton.h:109:1: 'assign' property of object type may become a dangling reference; consider using 'unsafe_unretained'`

Pull Request resolved: #1616

Reviewed By: jingping2015

Differential Revision: D26002203

Pulled By: ppansy

fbshipit-source-id: 5101f92f8f79b069f63b022129f70ad7becdb4e3

FBSDKLoginKit/FBSDKLoginKit/FBSDKLoginButton.h

@@ -106,7 +106,7 @@ NS_SWIFT_NAME(FBLoginButton)
   Gets or sets an optional nonce to use for login attempts. A valid nonce must be a non-empty string without whitespace.
  An invalid nonce will not be set. Instead, default unique nonces will be used for login attempts.
  */
-@property (assign, nonatomic, nullable) NSString *nonce;
+@property (copy, nonatomic, nullable) NSString *nonce;
 
 @end
 

FBSDKLoginKit/FBSDKLoginKit/FBSDKLoginButton.m

@@ -84,7 +84,7 @@ - (UIFont *)defaultFont
 - (void)setNonce:(NSString *)nonce
 {
   if ([FBSDKNonceUtility isValidNonce:nonce]) {
-    _nonce = nonce;
+    _nonce = [nonce copy];
   } else {
     _nonce = nil;
     [FBSDKLogger singleShotLogEntry:FBSDKLoggingBehaviorDeveloperErrors

FBSDKLoginKit/FBSDKLoginKitTests/LoginButtonTests.swift

@@ -21,7 +21,7 @@ import XCTest
 
 class LoginButtonTests: XCTestCase {
 
-  let validNonce: NSString = "abc123"
+  let validNonce: String = "abc123"
   var button: FBLoginButton! // swiftlint:disable:this force_unwrapping
   var sampleProfile: Profile {
     return Profile(