Fix occasional null verbose logging of request/response data in Objective-C

NSString's stringWithUTF8String: assumes that the const char * passed to it is
null-terminated, but NSData bytes does not guarantee that the data is terminated
by a null. This will occasionally cause stringWithUTF8String: to return nil instead
of the expected content.

http://stackoverflow.com/a/3384001 pointed the way to the solution for this.