Handle non-ascii characters in parameter values.

When embedding inline emoji, Outlook Web App generates headers that look
like this:

    Content-Type: image/png; name='OutlookEmoji-😊.png'

It looks like handling parametrized headers containing non-ASCII
characters was explicitly not supported previously. It's not clear to me
why. The parameter-splitting code works just fine with values like
these, though it would fail to correctly parse if e.g. the header name
contained non-ascii characters.

flanker/mime/message/headers/parsing.py

@@ -33,20 +33,15 @@ def parse_header(header):
 
 
 def parse_header_value(name, val):
-    if not is_pure_ascii(val):
-        if parametrized.is_parametrized(name, val):
-            raise DecodingError("Unsupported value in content- header")
-        return to_unicode(val)
-    else:
-        if parametrized.is_parametrized(name, val):
-            val, params = parametrized.decode(val)
-            if name == 'Content-Type':
-                main, sub = parametrized.fix_content_type(val)
-                return ContentType(main, sub, params)
-            else:
-                return WithParams(val, params)
+    if parametrized.is_parametrized(name, val):
+        val, params = parametrized.decode(val)
+        if name == 'Content-Type':
+            main, sub = parametrized.fix_content_type(val)
+            return ContentType(main, sub, params)
         else:
-            return val
+            return WithParams(val, params)
+    else:
+        return val if is_pure_ascii(val) else to_unicode(val)
 
 
 def is_empty(line):