Update asset_object::amount_to_string implementation

[abitmore]

Code copied from amount_from_string.

By the way, what if amount is negative?

[jmjatlanta]

Some surprising results. Some are due to the hack for the expected result, but the negative numbers are troubling. I didn't spend a lot of time debugging, but hopefully this will help.

https://gist.github.com/jmjatlanta/9c17db807cca638ef9158467592b8bf0

[abitmore]

@jmjatlanta I'd say don't use pow in test cases since it will introduce float points.

[jmjatlanta]

Please take a look at PR #1071. I fixed the POW and am just using hardcoded strings. Negative numbers still do not look right.

Update: Made a modification to asset_object::amount_to_string to handle negative numbers. Take a look. Could this method be passed a const?

Update2: switched to const reference.

[abitmore]

Thank you @jmjatlanta. It turns out that there were 2 bugs in the code.

• -12345 % 100 = -45, so fc::to_string(scaled_precision.value + decimals).erase(0,1) = to_string(100 + (-45)).erase(0,1) = to_string(55).erase(0,1) = "5", so final result is "-123.5"
• -12345 / 100000 = 0, so string result = fc::to_string(amount.value / scaled_precision.value) = "0", then after result += "." + ... the final result looks like a positive number.