Fix perf regression in the Tokenizer : avoid a concatenation

Version 2.3.1 :
-> % node bench2.js
htmlparser2:  01.86 ms/el

Version 3.1.2 without the fix :
-> % node tests/bench.js 
htmlparser2:  04.50 ms/el

Version 3.1.2 with the fix :
-> % node tests/bench.js
htmlparser2:  01.75 ms/el
Merge pull request #54 from abarre/master

[tokenizer] fix perf regression