Don't mutate options in `add` or `set`
Force add true and remove false flags in `add`, merge optional
Added missing word to emulateHTTP comment
Fix same mutation problem with existing/non-existing mix of models
Merge pull request #2539 from fixe/patch-1

Added missing word to emulateHTTP comment
Update PATCH/xhr check so that IE9 works

Fixes #2476
Merge pull request #2550 from jerryorr/master

Update PATCH/xhr check so that IE9 works