Fix AFConnectionOperation outputStream leak if no response is received

I noticed a memory leak when my app was auto-retrying an AFHTTPRequestOperation while a server was down - it seems that the outputStream instance for these operation objects are never dealloc'd when 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)addRequest {
    
    __weak ViewController *weakSelf = self;
	[self.http GET:@"/" parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {
        NSLog(@"Success ?? should not happen");
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"failure, requesting again");
        [weakSelf addRequest];
    }];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.http = [[AFHTTPRequestOperationManager alloc] initWithBaseURL:[NSURL URLWithString:@"http://bad.domain"]];
    [self addRequest];
}

@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)

I am not sure if this is an NSOutputStream bug or an apple documentation bug...