test: Add test for #979

Author: brson

src/test/run-pass/issue-979.rs

@@ -0,0 +1,12 @@
+resource r(b: @mutable int) {
+    *b += 1;
+}
+
+fn main() {
+    let b = @mutable 0;
+    {
+        let p = some(r(b));
+    }
+
+    assert *b == 1;
+}