Forced output format can sometimes be ignored when chained

[lovell]

As reported in #1156 (comment)

Use of the following:

sharp
  .png({
    force: true
  })
  .jpeg({
    force: false
  })

should produce PNG output for JPEG input, but instead produces JPEG output.

Need to modify the logic in the internal _updateFormatOut function to only update when force: true.

[lovell]

Commit 30ca424 adds the fix plus a test that would previously have failed.

[polarathene]

@lovell awesome thanks for this :)

I see that the v0.21.2 milestone is slowly growing with linked issues(all presently marked ready-to-ship), do you have any plans on when to release?(if it's a while away I guess I could build from the commit for now).

[lovell]

v0.21.2 now available.