title: how to change height between UITableView and Navigation Bar
body: <p>got a <code>UITableViewStyleGrouped</code>and <code>UITableViewStylePlain</code> by changes in storyboard, found that, top edge of plain table view sticks to navigation bar, while top gap in grouped style somehow because of header view. </p>
  
  <p><img src="https://i.stack.imgur.com/lbkdE.png" alt="enter image description here"></p>
  
  <p>But, as picture show, gap "a" is bigger than  "b", why? Are there any hidden elements around "a"? How to manage this gap so that it could be also stuck with bar? </p>
  
  <p>What's the default size of gap "a" and "b"? How to make "a" equal to "b", like "setting"</p>
  
  <p><img src="https://i.stack.imgur.com/Ax61U.png" alt="enter image description here"></p>
  
  <h3>below are my try-out</h3>
  
  <hr>
  
  <h3>tried set <code>heightForHeaderInSection:</code> and <code>viewForHeaderInSection:</code> like below</h3>
  
  <pre><code>- (CGFloat)tableView:(UITableView *)tableView heightForHeaderInSection:(NSInteger)section
  {
  return 0.0f;
  }
  
  - (UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section
  {
  UIView * header = [[UIView alloc] init];
  header.backgroundColor = [UIColor redColor];
  [self.view addSubview:header];
  
  return header;
  }
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/iZuyG.png" alt="enter image description here"></p>
  
  <hr>
  
  <h3>tried <code>heightForFooterInSection:</code> and <code>viewForFooterInSection:</code>, like below</h3>
  
  <pre><code>- (CGFloat)tableView:(UITableView *)tableView heightForFooterInSection:(NSInteger)section
  {
  return 0.0f;
  }
  
  - (UIView *)tableView:(UITableView *)tableView viewForFooterInSection:(NSInteger)section
  {
  UIView * footer = [[UIView alloc] init];
  footer.backgroundColor = [UIColor yellowColor];
  [self.view addSubview:footer];
  return footer;
  }
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/mwOtJ.png" alt="enter image description here"></p>
  
  <hr>
  
  <p>It looks neither of them work as expected, gap "a" is always there and not changed. </p>
  
  <p>The weird is, height of header and footer is still there, looks a minium height, </p>
  
  <h3>even if setting their height zero.</h3>