Better error msg if metadata contains a non us-ascii character

Pass 'é' or other non us-ascii character as a metadata value shows
a signature mismatch error. The reason is that S3 server ignores
those characters in headers since only US-ASCII characters are
supported in HTTP headers.

This commit shows an exception so users can understand better
what's going on.

minio/helpers.py

@@ -28,8 +28,12 @@
 # if math.ceil returns an integer and devide two integers returns a float, calculate
 # part size will cause errors, so make sure division integers returns a float.
 from __future__ import division
-import io
 
+from __future__ import unicode_literals
+# future_str is unicode or str in both Python 2 and 3
+from builtins import str as future_str
+
+import io
 import collections
 import base64
 import hashlib
@@ -654,6 +658,16 @@ def optimal_part_info(length, part_size):
 def amzprefix_user_metadata(metadata):
     m = dict()
     for k,v in metadata.items():
+       # Check if metadata value has US-ASCII encoding since it is
+       # the only one supported by HTTP headers. This will show a better
+       # exception message when users pass unsupported characters
+       # in metadata values.
+       try:
+            if isinstance(v, future_str):
+                 v.encode('us-ascii')
+       except UnicodeEncodeError:
+            raise ValueError('Metadata supports only US-ASCII characters.')
+
        if is_amz_header(k) or is_supported_header(k) or is_storageclass_header(k):
             m[k] = v
        else:

setup.py

@@ -40,6 +40,7 @@
 ]
 
 requires = [
+    'future',
     'urllib3',
     'pytz',
     'certifi',