"let" expression strips expected type during parsing

[ChrisLoer]

This showed up in a bug report as a somewhat cryptic "Type string is not interpolatable" error for a style property of this form:

"fill-color": ["let", 
   "someNumber",
   500,
   [
          "interpolate",
          ["linear"],
          ["var", "someNumber"],
          274,
          "#edf8e9",
          1551,
          "#006d2c"
    ]

The same interpolate expression without using a "let/var" worked just as expected. It turns out the reason was that the color strings (eg "#edf8e9" were getting implicitly coerced to the "color" type because that's the expected type for the fill-color property. Inside the let expression though, the implicit coercion didn't happen.

I think this is just a mistake here:

mapbox-gl-js/src/style-spec/expression/definitions/let.js

Line 52 in a1233cb

const result = context.parse(args[args.length - 1], args.length - 1, undefined, bindings);

That undefined should actually be context.expectedType, in order to correctly set the let expression's result type. It looks like we're already doing that in native:

https://github.com/mapbox/mapbox-gl-native/blob/a991eec597541c32682946b541e22ccad0f98dd1/src/mbgl/style/expression/let.cpp#L61

cc @malwoodsantoro @mollymerp @ryanhamley