Fix AFConnectionOperation outputStream leak if no response is received

I noticed a memory leak when my app was retrying an AFHTTPRequestOperation while a server was down - it seems that the outputStream instance for these operation objects are never dealloc'd if no response is received.

Here is a minimal snippet to drop into a single-page-view-controller project to reproduce:

```objective-c
#import "AFNetworking.h"
#import "ViewController.h"

@interface ViewController ()
@property (nonatomic) AFHTTPRequestOperationManager *http;
@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSURL *failingURL = [NSURL URLWithString:@"http://127.0.0.1:3456"];
    self.http = [[AFHTTPRequestOperationManager alloc] initWithBaseURL:failingURL];
    [self doFailingRequest];
}

- (void)doFailingRequest
{
    __weak ViewController *weakSelf = self;
	[self.http GET:@"/" parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {
        NSLog(@"Success ?? should not happen in this example");
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"Failure, requesting again");
        [weakSelf doFailingRequest];
    }];
}

@end
```

Even though the apple documentation states that calling 'close' should implicitly unschedule an NSStream from its run loop, apparently this doesn't happen if the stream was never 'open'.

In the current codebase the outputStream is only open if a response is received - I moved this call to open the stream as soon as it is scheduled on a run loop.

##### Before the fix:
![before_fix](https://cloud.githubusercontent.com/assets/380187/3475434/f518ee48-02f0-11e4-805e-e8f019bce9ce.png)

##### After the fix:
![after_fix](https://cloud.githubusercontent.com/assets/380187/3475435/fa318bce-02f0-11e4-83bc-e356c3aa4838.png)