filters with collator expressions incorrectly treated as legacy filters

[ChrisLoer]

The isExpressionFilter check doesn't know about the possible collator expression:

mapbox-gl-js/src/style-spec/feature_filter/index.js

Lines 25 to 31 in 01f0857

	case '==':
	case '!=':
	case '>':
	case '>=':
	case '<':
	case '<=':
	return filter.length === 3 && (Array.isArray(filter[1]) || Array.isArray(filter[2]));

It should also support an optional case in which filter.length === 4 because a collator is being passed in.

I haven't tested yet, but I believe the result of this is that if you make a filter using an expression with a collator, it will incorrectly decide it's a legacy filter and attempt to convert it (which won't work because we don't support collators in legacy filters).

/cc @1ec5 @anandthakker