Bugfix on convert_number (impacts at least _resize and _crop_resize)

Depending on php configuration, float are sometimes converted to strings using commas instead of points. This notation can create issues since the conversion from string to float will return an integer.
For instance: "1.2" / 10 == 0.12 but "1,2" / 10 == 0.1...
Merge pull request #1571 from sdrdis/patch-2

Bugfix on convert_number (impacts at least _resize and _crop_resize)