Merge pull request #127 from Shopify/bugfix/116-customerID-type

Change customerID type from NSString to the correct type - NSNumber.

Mobile Buy SDK/Mobile Buy SDK/Models/BUYCheckout.h

@@ -237,7 +237,7 @@
 /**
  *  Customer ID associated with the checkout
  */
-@property (nonatomic, copy, readonly) NSString *customerId;
+@property (nonatomic, copy, readonly) NSNumber *customerId;
 
 /**
  *  An optional note attached to the order

Mobile Buy SDK/Mobile Buy SDK/Models/BUYCheckout_Private.h

@@ -48,7 +48,7 @@
 @property (nonatomic, copy) NSDate *updatedAtDate;
 @property (nonatomic, strong) BUYMaskedCreditCard *creditCard;
 @property (nonatomic, strong) BUYOrder *order;
-@property (nonatomic, copy) NSString *customerId;
+@property (nonatomic, copy) NSNumber *customerId;
 @property (nonatomic, strong) NSURL *privacyPolicyURL;
 @property (nonatomic, strong) NSURL *refundPolicyURL;
 @property (nonatomic, strong) NSURL *termsOfServiceURL;