generators: make toPretty handle floats correctly

lib/generators.nix

@@ -143,6 +143,7 @@ rec {
   }@args: v: with builtins;
     let     isPath   = v: typeOf v == "path";
     in if   isInt      v then toString v
+    else if isFloat    v then "~${toString v}"
     else if isString   v then ''"${libStr.escape [''"''] v}"''
     else if true  ==   v then "true"
     else if false ==   v then "false"

lib/tests/misc.nix

@@ -369,6 +369,7 @@ runTests {
   testToPretty = {
     expr = mapAttrs (const (generators.toPretty {})) rec {
       int = 42;
+      float = 0.1337;
       bool = true;
       string = ''fno"rd'';
       path = /. + "/foo";
@@ -381,6 +382,7 @@ runTests {
     };
     expected = rec {
       int = "42";
+      float = "~0.133700";
       bool = "true";
       string = ''"fno\"rd"'';
       path = "/foo";