Actually respect the requested human-readable precision.

--- src/string_util.cc
+++ src/string_util.cc
@@ -55,6 +61,8 @@ void ToExponentAndMantissa(double val, double thresh, int precision,
     for (size_t i = 0; i < arraysize(kBigSIUnits); ++i) {
       scaled /= one_k;
       if (scaled <= big_threshold) {
+        mantissa_stream << std::fixed;
+        mantissa_stream.precision(precision);
         mantissa_stream << scaled;
         *exponent = i + 1;
         *mantissa = mantissa_stream.str();
