don't return utf8 header in example

Since the updated RFC 7230 implys that new Headers Key and Value should be
sent as USASCII only don't try to test utf8 headers in examples.

We now only encode them to ascii. Gunicorn will fail if it's unable to encode
them letting the responsability to the application to correctly encode the
response. (we are just a gateway).

While i'm here simplify the code to not create an extra function only used at
one place.

NOTE: if anyone come to a better solution, i am happy to revisit it on the
next release.

fix #1151

examples/test.py

@@ -21,7 +21,7 @@ def app(environ, start_response):
         ('Content-type', 'text/plain'),
         ('Content-Length', str(len(data))),
         ('X-Gunicorn-Version', __version__),
-        ("Test", "test тест"),
+        #("Test", "test тест"),
     ]
     start_response(status, response_headers)
     return iter([data])

gunicorn/http/wsgi.py

@@ -320,7 +320,7 @@ def send_headers(self):
         tosend.extend(["%s: %s\r\n" % (k, v) for k, v in self.headers])
 
         header_str = "%s\r\n" % "".join(tosend)
-        util.write(self.sock, util.to_latin1(header_str))
+        util.write(self.sock, util.to_bytestring(header_str, "ascii"))
         self.headers_sent = True
 
     def write(self, arg):

gunicorn/util.py

@@ -25,7 +25,6 @@
 from gunicorn.six import text_type
 from gunicorn.workers import SUPPORTED_WORKERS
 
-
 MAXFD = 1024
 REDIRECT_TO = getattr(os, 'devnull', '/dev/null')
 
@@ -498,23 +497,14 @@ def check_is_writeable(path):
     f.close()
 
 
-def to_bytestring(value):
-    """Converts a string argument to a byte string"""
-    if isinstance(value, bytes):
-        return value
-    if not isinstance(value, text_type):
-        raise TypeError('%r is not a string' % value)
-    return value.encode("utf-8")
-
-
-def to_latin1(value):
+def to_bytestring(value, encoding="utf8"):
     """Converts a string argument to a byte string"""
     if isinstance(value, bytes):
         return value
     if not isinstance(value, text_type):
         raise TypeError('%r is not a string' % value)
-    return value.encode("latin-1")
 
+    return value.encode(encoding)
 
 def warn(msg):
     print("!!!", file=sys.stderr)