use implicit string conversion instead of String constructor

[marijaselakovic]

Hi,
I would propose few optimizations:

1. Use implicit string conversion, instead of String constructor. Especially if the expected input is string, like in functions: capitalize, downcase, upcase, truncate, truncate_words and replace (jsperf test: http://jsperf.com/implicitstringconversion)

2. Instead of js.substr(0,1), use equivalent js[0].

[limitLam]

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