fix the rounding bug in roundShortest method

[dreamerjackson]

optimization roundShortest method for effectiveness
which had apply in go source code: go-source-strconv

[mwoss]

Thanks for your contribution @dreamerjackson. Before diving in code review, could you provide proper benchmarks too? So all be aware of the importance of this optimization.

[dreamerjackson]

@mwoss soory, i check it and find it's about a bug in rounding shortest. the fix and detail description is there:go-commit
i add the test in Most recent commit

[dreamerjackson]

@cmars @chrismrivera @soyangel the fix can merge?

[mwoss]

Hi @dreamerjackson. I kinda forgot about your PR, I apologize. Could you rename the pull request title to match code content? As you said it's about fixing the bug, not optimization at all. Below I provided the additional benchmarks.

goos: windows
goarch: amd64
pkg: github.com/shopspring/decimal

Benchmark name	Loops	Time per op	Memory per op	Alloc per op
BenchmarkNewFromFloat-8	2350124	506 ns/op	48 B/op	2 allocs/op
BenchmarkNewFromFloatBugFix-8	2228163	539 ns/op	48 B/op	2 allocs/op

[dreamerjackson]

@mwoss i have fixed the title description，please review it again , thx~

[mwoss]

Properly vendorder, ready to merge.