Fix floating point bug in encodeGrid
Fix floating point bug in encodeGrid

Port of https://github.com/google/open-location-code/commit/0ec5cfcc9198cca44de698a8f653391037195b69#diff-0a717cb928d23561819cf0be1b3f75ed

Fixes encoding test failure:
```
6FH32222+222,1,1,1,1,1.000025,1.00003125
```

google/open-location-code-swift,google/open-location-code-swift,google/open-location-code-swift