[Sass] [Indented] Allow quotes in indented-syntax @import.

Closes gh-118

doc-src/INDENTED_SYNTAX.md

@@ -121,10 +121,7 @@ is compiled to:
 
 ### `@import`
 
-The `@import` directive in Sass does not require quotes.
-Quotes may be used;
-if they are, the directive will always be interpreted
-as a plain CSS `@import` rule.
+The `@import` directive in Sass does not require quotes, although they may be used.
 For example, this SCSS:
 
     @import "themes/dark";

doc-src/SASS_CHANGELOG.md

@@ -45,6 +45,12 @@ this won't cause any backwards-incompatibilities.
 * `#{}` interpolation within strings will now be converted properly
   to `#{}` interpolation within strings.
 
+### `@import` in Sass
+
+The Sass `@import` statement now allows non-CSS files to be specified with quotes,
+for similarity with the SCSS syntax. For example, `@import "foo.sass"`
+Will now import the `foo.sass` file, rather than compiling to `@import "foo.sass";`.
+
 ### CSS Parsing
 
 The proprietary Microsoft `alpha(opacity=20)` syntax is now correctly parsed.

lib/sass/engine.rb

@@ -461,10 +461,13 @@ WARNING
 
       # If value begins with url( or ",
       # it's a CSS @import rule and we don't want to touch it.
-      if directive == "import" && value !~ /^(url\(|")/
+      if directive == "import" && value !~ /^(url\(|["'])/
         raise SyntaxError.new("Illegal nesting: Nothing may be nested beneath import directives.",
           :line => @line + 1) unless line.children.empty?
-        value.split(/,\s*/).map {|f| Tree::ImportNode.new(f)}
+        value.split(/,\s*/).map do |f|
+          f = $1 || $2 || $3 if f =~ Sass::SCSS::RX::STRING || f =~ Sass::SCSS::RX::URI
+          Tree::ImportNode.new(f)
+        end
       elsif directive == "mixin"
         parse_mixin_definition(line)
       elsif directive == "include"

test/sass/templates/import.sass

@@ -3,7 +3,7 @@ $preconst: hello
 =premixin
   pre-mixin: here
 
-@import importee.sass, scss_importee, basic.sass, basic.css, ../results/complex.css, partial.sass
+@import importee.sass, scss_importee, "basic.sass", basic.css, ../results/complex.css, url(partial.sass)
 
 nonimported
   :myconst $preconst