Modify chewing_set_maxChiSymbolLen to fix #50

The original chewing_set_maxChiSymbolLen adds ZUIN_SIZE when determine
if auto commit is needed. It is ambiguous because user expects the value
set by chewing_set_maxChiSymbolLen is the maximum character/symbol in
preedit buffer. By removing ZUIN_SIZE, the behavior is the same as user
expectation.