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correct the fixed-mode halfway example in the dtoa headers - #332
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The FIXED-mode comments in double-to-string.h, fixed-dtoa.h and bignum-dtoa.h illustrate the halfway rule with toFixed(0.15, 2) returning buffer "2", point 0. The library returns "15", point 0, and 0.15 is not a halfway case for a double at any digit count: the nearest double lies below the tie, so one digit gives "1" (the ToFixed tests pin "0.1"). Use the exactly representable toFixed(0.25, 1) -> "3", point 0 instead and pin it at the FastFixedDtoa and BignumDtoa layers.
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Repro: DoubleToAscii(0.15, FIXED, 2), FastFixedDtoa(0.15, 2) and BignumDtoa(0.15, BIGNUM_DTOA_FIXED, 2) return buffer "15", point 0, not the documented buffer "2", point 0.
Cause: 0.15 is not a halfway case for a double: the nearest value is 0.1499999999999999944..., below the tie, so even one requested digit yields "1" (the existing ToFixed test pins "0.1"), and with two digits nothing is cut at a tie.
Fix: illustrate the rule with the exactly representable halfway case toFixed(0.25, 1) -> buffer "3", point 0 in all three headers, and pin it with checks at the FastFixedDtoa and BignumDtoa layers, matching the ToFixed(0.25, 1) -> "0.3" case already in test-conversions.cc.