Python 2 compatibility with unicode response header keys and wsgiref

[mircea3]

Example:

from __future__ import unicode_literals

...
response.headers['Cache-Control'] = 'max-age=0'
...

Results in:

Traceback (most recent call last):
  File "bottle.py", line 1134, in wsgi
    start_response(response._wsgi_status_line(), response.headerlist, exc_info)
  File "/usr/lib/python2.7/wsgiref/handlers.py", line 179, in start_response
    assert type(name) is StringType,"Header names must be strings"
AssertionError: Header names must be strings

This is similar to issue #923, including the fix, but applies to the header key, as opposed to the value.

The simple fix previously used for header value:

def _hval(value):
+    value = tonat(value)
     ...

could also be used for the header key:

def _hkey(key):
+   key = tonat(key)
    ...