The logic in our double-based sprintf logic attempts to round
values manually by inspecting a long-form version of the double.
If we need to round, and the next digit is a five, it will round
toward zero (truncate) iff that five is the last digit in the long
unrounded string. This appears to have been an attempt to mimic
C printf's behavior of rounding "true half" to even in the
presence of inaccurately-represented IEEE754 decimals.

This commit changes the pre-formatting to actually format with the
specified precision, allowing NumberFormat's default HALF_EVEN
logic to to the work for us.

Fixes #4157.

Format initial string to specific precision, so rounding works.