Fixed: insertAttributedString... does not coalesce

Before this patch, inserting attributedStrings into attributedStrings always inserted a new attributes range, even if the attributes are identical to the range inserted into. This is e.g. common with the new text object that inserts the current typing attributes together with each character. This eventually results in one div per character and is not good for performance.
This patch coalesces the attributes as cocoa does.
Best greetings,
Daniel