Auto merge of #19490 - oli-obk:json_non_string_key_maps, r=alexcrichton

importing object type string key maps is still supported
writing them should be explicit, and can be done as follows

```rust
let some_tree_map : TreeMap<String, Json> = ...;
Json::Object(some_tree_map).to_writer(&mut writer);
```

related to #8335, #9028, #9142

Author: bors

src/librbml/lib.rs

@@ -1059,8 +1059,8 @@ pub mod writer {
             self.end_tag()
         }
 
-        fn emit_map_elt_key<F>(&mut self, _idx: uint, mut f: F) -> EncodeResult where
-            F: FnMut(&mut Encoder<'a, W>) -> EncodeResult,
+        fn emit_map_elt_key<F>(&mut self, _idx: uint, f: F) -> EncodeResult where
+            F: FnOnce(&mut Encoder<'a, W>) -> EncodeResult,
         {
 
             try!(self.start_tag(EsMapKey as uint));