Fix incorrect documentation of map-has-key

`map-has-key` was previously documented as taking a single parameter, `$key`. Since this is obviously not the case, I updated the documentation to show `map-has-key` taking two parameters, `$map` and `$key`.
Do not freeze `true` or `false` in `Script::Value::Base.new`.

This fixes #1210, wherein the freezing of `true` causes strange side
effects in other gems. It also separates the `value.freeze` call from
the assignment `@value = value`, to emphasize that `value.freeze` is a
side effect that mutates the value, not a function that returns a frozen
copy of the value.
Merge pull request #1204 from penman/stable

Fix incorrect documentation of map-has-key
Avoid unnecessary interpolation in `sass-convert`.

Closes #1208
More efficient checks for freezability.
Add a changelog entry.
Merge branch 'dont-freeze-true-false' into stable

Conflicts:
	doc-src/SASS_CHANGELOG.md

Closes #1210
Bundle the canonical version of listen rather than my fork.

The canonical version has merged in the patch in my fork that we rely
on.
Merge branch 'stable' of github.com:nex3/sass into stable
Bump VERSION to 3.3.5.
Merge branch 'stable'

Conflicts:
	VERSION
	doc-src/SASS_CHANGELOG.md