fix display(d, m, x) where m expects a json string

Fix queryverse/VegaLite.jl#127
After the fix the following works out of the box in new versions of jupyterlab, 
generating an interactive VegaLite visualization,
without additional packages except Ijulia. 
```
spec = raw"""
{
  "data": {
    "values": [
      {"a": "A","b": 28}, {"a": "B","b": 55}, {"a": "C","b": 43},
      {"a": "D","b": 91}, {"a": "E","b": 81}, {"a": "F","b": 53},
      {"a": "G","b": 19}, {"a": "H","b": 87}, {"a": "I","b": 52}
    ]
  },
  "mark": "point",
  "encoding": {
    "x": {"field": "a", "type": "ordinal"},
    "y": {"field": "b", "type": "quantitative"}
  }
}
"""
display("application/vnd.vegalite.v2+json", spec)
```

Author: cnliao

src/inline.jl

@@ -47,14 +47,27 @@ function limitstringmime(mime::MIME, x)
     return String(take!(buf))
 end
 
+const ipy_mime_json = [
+    "application/vnd.dataresource+json",
+    "application/vnd.vegalite.v2+json",
+    "application/vnd.vega.v3+json",
+]
+_display_dict(m::MIME, m_str, x) = Dict(m_str=>limitstringmime(m, x))
+# escape JSON string correctly before send_ipython
+for mime in ipy_mime_json
+    @eval begin
+        _display_dict(m::MIME{Symbol($mime)}, m_str, x) = Dict(m_str=>JSON.JSONText(limitstringmime(m, x)))
+    end
+end
+
 for mime in ipy_mime
     @eval begin
         function display(d::InlineDisplay, ::MIME{Symbol($mime)}, x)
             send_ipython(publish[],
                          msg_pub(execute_msg, "display_data",
                                  Dict(
                                   "metadata" => metadata(x), # optional
-                                  "data" => Dict($mime => limitstringmime(MIME($mime), x)))))
+                                  "data" => _display_dict(MIME($mime), $mime, x))))
         end
         displayable(d::InlineDisplay, ::MIME{Symbol($mime)}) = true
     end