ascii-error on train-endpoint
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**rasa NLU version**: `0.9.1`

**Used backend / pipeline** (`mitie`, `spacy_sklearn`, ...): `spacy_sklearn`

**Operating system** (windows, osx, ...): `osx`

**Issue**:
When I post a json-file with expressions with unicode characters (e.g. euro sign (€)) to the servers' /train endpoint, I get an "500 Internal Server Error" response and the server-logs say:
`UnicodeEncodeError: 'ascii' codec can't encode character u'\\u20ac' in position 72: ordinal not in range(128)`

My steps:
`expressions.json`:
```json
{
  "rasa_nlu_data": {
    "common_examples": [
      {
        "text": "50 €",
        "intent": "set_price",
        "entities": []
      }
    ]
  }
}
```
- `curl -XPOST localhost:5000/train -d @expressions.json`
```
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<title>500 Internal Server Error</title>
<h1>Internal Server Error</h1>
<p>The server encountered an internal error and was unable to complete your request.  Either the server is overloaded or there is an error in the application.</p>
```

**Content of configuration file** (if used & relevant):
```json

```

diff --git a/rasa_nlu/data_router.py b/rasa_nlu/data_router.py
index 83f1899449c..31d9b9213ca 100644
--- a/rasa_nlu/data_router.py
+++ b/rasa_nlu/data_router.py
@@ -14,6 +14,7 @@ import io
 
 from builtins import object
 from typing import Text
+from future.utils import PY3
 
 from concurrent.futures import ProcessPoolExecutor as ProcessPool
 from twisted.internet.defer import Deferred, maybeDeferred
@@ -216,8 +217,12 @@ class DataRouter(object):
         }
 
     def start_train_process(self, data, config_values):
-        f = tempfile.NamedTemporaryFile("w+", suffix="_training_data.json", delete=False)
-        f.write(data)
+        if PY3:
+            f = tempfile.NamedTemporaryFile("w+", suffix="_training_data.json", delete=False, encoding="utf-8")
+            f.write(data)
+        else:
+            f = tempfile.NamedTemporaryFile("w+", suffix="_training_data.json", delete=False)
+            f.write(data.encode("utf-8"))
         f.close()
         # TODO: fix config handling
         _config = self.config.as_dict()
['rasa_nlu/data_router.py']