Parse SDK 0.2.15 is blocking main thread when preparing to upload a PFObject with lots of data. Moving this to a background thread avoids UI freeze

PFRequest_0.2.15_snippet.m

// original file: PFRequest.m
- (void)buildAndSignPostBody {
    for (NSString *key in self.command.params) {
        [self setPostValue:[self.command.params objectForKey:key] forKey:key];
    }    
    
    // This also ensures that posts are not empty, which would break various things
    [self setPostValue:[NSString stringWithFormat:@"i%@", PARSE_VERSION] forKey:@"v"];
    
    
    __block NSString *header = nil;
    
    /* I'm using DISPATCH_QUEUE_PRIORITY_LOW, as DISPATCH_QUEUE_PRIORITY_BACKGROUND
     * is not available until 4.3 
     */
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        // Background Thread
        [self buildPostBody];
        
        header = PF_OAuthorizationHeader([self url],
                                         [self requestMethod], 
                                         [self postBody], 
                                         APPLICATION_ID, 
                                         CLIENT_KEY, 
                                         nil,
                                         nil);
        dispatch_sync(dispatch_get_main_queue(), ^{
            // Main Thread
            [self addRequestHeader:@"Authorization" value:header];
        });
        
    }); 
    
}