Add specs for issue 1583

This PR add specs for sass/libsass#1583

Author: xzyfer

spec/libsass-todo-issues/issue_1583/expected.compact.css

@@ -0,0 +1 @@
+foo { string: (foo,); length: 6; }

spec/libsass-todo-issues/issue_1583/expected.compressed.css

@@ -0,0 +1 @@
+foo{string:(foo,);length:6}

spec/libsass-todo-issues/issue_1583/expected.expanded.css

@@ -0,0 +1,4 @@
+foo {
+  string: (foo,);
+  length: 6;
+}

spec/libsass-todo-issues/issue_1583/expected_output.css

@@ -0,0 +1,3 @@
+foo {
+  string: (foo,);
+  length: 6; }

spec/libsass-todo-issues/issue_1583/input.scss

@@ -0,0 +1,4 @@
+foo {
+  string: inspect(&);
+  length: str-length(inspect(&));
+}