Tests: amount passed as string

Enforce behavior is same for identical value passed as string or int.
Fix: Money sign for string amounts was negative

Typed comparison is not necessary because of number_format check and
produces bug.
Tests: removed irrelevant 0 from decimals_invalid
Tests: fixed expected value for decimalsForStringAmount_invalid
Merge pull request #7 from Kdyby/bug_string_amount

Bug: Amount passed as string has always negative sign