Small issue on subtraction

[takanuva]

The operator -= isn't working properly (should be easy to reproduce). On lens.ml, line 173:

let (-=) l v = _modify ((-) v) l

(This probably affects line 183 as well.)

The value being subtracted (v) is being used on the left position on the subtraction, giving an incorrect result. Could be changed to, e.g., ((+) (-v)) or (fun x -> x - v).

[pdonadeo]

Sorry for the delay, I missed the notification.

I'll fix and release ASAP.

[pdonadeo]

@takanuva 1.2.2 just released and I sent the pull request to the OPAM repository.