fix PostPolicy bool operator condition (#137)

Author: ijoseph

include/miniocpp/args.h

@@ -570,7 +570,7 @@ struct PostPolicy {
 
   ~PostPolicy() = default;
 
-  explicit operator bool() const { return !bucket.empty() && !expiration_; }
+  explicit operator bool() const { return !bucket.empty() && expiration_; }
 
   error::Error AddEqualsCondition(std::string element, std::string value);
   error::Error RemoveEqualsCondition(std::string element);