closed issue#4428:fixed label display incorrect if invoking getLetter and it's multi-line.
change access specifier of SpriteBatchNode::addSpriteWithoutQuad.
Add a test case to reproduce issue 4428.
update test case of issue 4428.
fixed typo.
Merge pull request #5843 from Dhilan007/develop_label

closed #4428: fixed display incorrect of multi-line label if invoking 'getLetter'.