This commit fixes bug in Iterator.assertInteger method if the next
conditions are met:
- Iterator reads data from `io.Reader`,
- expected value is `0` (zero)
- `Iterator.tail == Iterator.head + 1`
- `Iterator.tail < len(Iterator.buf)`
- value in the buffer after `Iterator.tail` is presented from the previous read and has '.' character.
Typical error which user cal see is:
- assertInteger: can not decode float as int, error found in #X byte of ...
Regression test added for checking the correct behaviour.
Fixes#476
