hash to json strangeness

Nobody has claimed this yet.

I've been struggling with a hash to json conversion that fails but only some of the time.

the output from mirb is correct

```
mirb> JSON.stringify({:id=>"c58", :method=>"vm.add", :params=>{:name=>"eval1", :url=>"https://gist.github.com/donpdonp/aab0987a4e062ca66d6a"}})
=> "{\"id\":\"c58\",\"method\":\"vm.add\",\"params\":{\"name\":\"eval1\",\"url\":\"https://gist.github.com/donpdonp/aab0987a4e062ca66d6a\"}}"
```

when I stringify the same object in mruby inside the C app, the output is wrong.
the ruby creating the output is at
https://gist.github.com/donpdonp/07bcb50f4f61680962d3#file-vm-rb-L104
notice the "name" : "eval1 url...."

``` ruby
vm to_s {:id=>"c58", :method=>"vm.add", :params=>{:name=>"eval1", :url=>"https://gist.github.com/donpdonp/aab0987a4e062ca66d6a"}}
vm stringify {"id":"c58","method":"vm.add","params":{"name":"eval1 https://gist.github.com/donpdonp/aab0
987a4e062ca66d6a","url":"https://gist.github.com/donpdonp/aab0987a4e062ca66d6a"}}
```

If I add a debug print statement like this

``` diff
diff --git a/src/mrb_json.c b/src/mrb_json.c
index a04871a..1262748 100644
--- a/src/mrb_json.c
+++ b/src/mrb_json.c
@@ -95,6 +95,7 @@ mrb_value_to_string(mrb_state* mrb, mrb_value value) {
mrb_str_concat(mrb, str, enckey);
mrb_str_cat_cstr(mrb, str, ":");
obj = mrb_hash_get(mrb, value, key);
+ printf("js k:%s v:%s\n", mrb_string_value_cstr(mrb, &key), mrb_string_value_cstr(mrb, &obj));
mrb_str_concat(mrb, str, mrb_value_to_string(mrb, obj));
if (n != l - 1) {
mrb_str_cat_cstr(mrb, str, ",");
```

The output becomes correct where "name": "eval1" without the url

```
vm to_s {:id=>"eys", :method=>"vm.add", :params=>{:name=>"eval1", :url=>"https://gist.github.com/donpdon
p/aab0987a4e062ca66d6a"}}
js k:id v:eys
js k:method v:vm.add
js k:params v:{:name=>"eval1", :url=>"https://gist.github.com/donpdonp/aab0987a4e062ca66d6a"}
js k:name v:eval1
js k:url v:https://gist.github.com/donpdonp/aab0987a4e062ca66d6a
vm stringify {"id":"eys","method":"vm.add","params":{"name":"eval1","url":"https://gist.github.com/donpd
onp/aab0987a4e062ca66d6a"}}
```

I apologize for the complicated setup and for filing a bug when the mirb output is correct, but I cant think of anywhere else to look for this problem. Do you have any ideas here? thanks.

Start with src/mrb_json.c, especially mrb_value_to_string, and reproduce the difference between mirb and the embedded mruby C application using the hash shown in the issue. Compare the serialized output with and without the temporary debug print; done means the embedded output preserves separate name and url values without relying on the print statement.

Written by the indexing model from the issue text.