[mutator] setIfMissing messes with squashing

[simen]

The mutator considers "setIfMissing" a non-squashable patch since it conceptually could change the shape of the document. This messes up squashing since Sanity always sends a set if missing patch before each delta patch. It is conceptually correct to squash setIfMissing patches as long as the resulting patch contains one of each setIfMissing when it first arrive.

[setIfMissing("field"), diffMatchPatch("field", ...), setIfMissing("field"), diffMatchPatch("field", ...), setIfMissing("field"), diffMatchPatch("field", ...)] can be squashed to [setIfMissing("field"), diffMatchPatch("field", ...)]