Various small fixes

- Correct polyline encoding bug for small negative delta (when float c is negative but rounded binary is 0, ~0 = FFFF...)
- Polyline encoding: use proper rounding for float to binary conversion, this allow polyline tests copied over from the web to pass properly (also correct few previous tests)
- Add distance to walk legs in JSON output, as it seems to be mandatory for existing OTP clients
- Remove duplicated initializations in request buffer, zero out the buffer to zero the padding bytes and to make it safer in case somebody add new fields
- Output NULL instead of "null" string for plan error when no error to make JSON output compatible with OTP
- Replace inexisting (removed?) function in hashgrid unit function.