Fix regression in false positive caching of sass_file()

[cpsievert]

Closes #84

[cpsievert]

When we recently started having as_sass() return HTML deps (#74), that should have broken this test expectation

sass/tests/testthat/test-cache.R

Lines 55 to 74 in 489f545

	# Make sure cache key doesn't change with new/temporary HTML dependencies
	my_layer <- function() {
	src <- tempfile()
	dir.create(src)
	sass_layer(
	"@function fib($x) {
	@if $x <= 1 {
	@return $x
	}
	@return fib($x - 2) + fib($x - 1);
	}
	body { width: fib(27);}",
	html_deps = htmltools::htmlDependency("foo", "1.0", src)
	)
	}
	expect_cached(
	my_layer(),
	"body{width:196418;}"
	)

Turns out that test wasn't working the way I intended it to work when I added it in #63, so
d1b55e3 fixes that issue

[wch]

Note that this won't remove an html dependency if it's included as a child of a tag object (as opposed to being added with attachDependency()):

x <- div(
  "hello",
  htmlDependency("x", "1.0", c(src = "abc"))
)
htmlDependencies(x) <- NULL
findDependencies(x)
#> [[1]]
#> List of 10
#>  $ name      : chr "x"
#>  $ version   : chr "1.0"
#>  $ src       :List of 1
#>   ..$ src: chr "abc"
#>  $ meta      : NULL
#>  $ script    : NULL
#>  $ stylesheet: NULL
#>  $ head      : NULL
#>  $ attachment: NULL
#>  $ package   : NULL
#>  $ all_files : logi TRUE
#>  - attr(*, "class")= chr "html_dependency"

Same is true if it's an element in a tagList or list:

x <- tagList(
  div("hello"),
  htmlDependency("x", "1.0", c(src = "abc"))
)
htmlDependencies(x) <- NULL
findDependencies(x)
#> [[1]]
#> List of 10
#>  $ name      : chr "x"
#>  $ version   : chr "1.0"
#>  $ src       :List of 1
#>   ..$ src: chr "abc"
#>  $ meta      : NULL
#>  $ script    : NULL
#>  $ stylesheet: NULL
#>  $ head      : NULL
#>  $ attachment: NULL
#>  $ package   : NULL
#>  $ all_files : logi TRUE
#>  - attr(*, "class")= chr "html_dependency"

[cpsievert]

I don't think this is an actual issue since as_sass() currently doesn't have a method for tags for dependencies (this is partially the motivation for having the sass_layer(html_deps)):

> sass::as_sass(list(".foo{}", htmltools::htmlDependency("foo", "1.0", ".")))
Error in as_sass_.default(val) : 
  as_sass() not implemented for object of class: 'html_dependency'

That being said, maybe it still make sense to add a case:

} else if (inherits(x, "html_dependency")) {
    x <- NULL
  }

just in case we decide to add a no-op method someday