lib.isStorePath: fix `false` result when passed a path object

Since `isStorePath` relies on comparing against builtins.storeDir
(a string), we need to convert the input into a string as well.

lib/strings.nix

@@ -502,9 +502,12 @@ rec {
        => false
   */
   isStorePath = x:
-       isCoercibleToString x
-    && builtins.substring 0 1 (toString x) == "/"
-    && dirOf x == builtins.storeDir;
+    if isCoercibleToString x then
+      let str = toString x; in
+      builtins.substring 0 1 str == "/"
+      && dirOf str == builtins.storeDir
+    else
+      false;
 
   /* Convert string to int
      Obviously, it is a bit hacky to use fromJSON that way.

lib/tests/misc.nix

@@ -112,7 +112,7 @@ runTests {
         storePathAppendix = isStorePath
           "${goodPath}/bin/python";
         nonAbsolute = isStorePath (concatStrings (tail (stringToCharacters goodPath)));
-        asPath = isStorePath goodPath;
+        asPath = isStorePath (/. + goodPath);
         otherPath = isStorePath "/something/else";
         otherVals = {
           attrset = isStorePath {};