fix(encoding): Fix double-encoding of HTTP response (#220)

ecba3ea added a call to encode
the response body before sending it to the server if the body is
of type str.

This is the correct behavior for python 3, where str represents a
an unencoded string, but is incorrect for python 2. In python 2,
str represents an already-encoded string and unicode represents
an unencoded string.

This is breaking for prometheus monitoring, which is sending already-
encoded bytes to the respond() function. In python 2, those have type
str, so we're trying to re-encode them. This works fine as long as there
are only ASCII characters in the string, but breaks otherwise.

In order to support both python 2 and 3, try to use 'unicode' (as
exists in 2) and if it's absent assign str to unicode.

spinnaker-monitoring-daemon/spinnaker-monitoring/http_server.py

@@ -28,7 +28,13 @@
       HTTPServer,
       BaseHTTPRequestHandler)
   from urllib.request import unquote as urllibUnquote
-
+
+# The unicode type only exists in python 2; it has been renamed str in python 3
+# In order to support python 2 and 3, alias unicode to str if it's not present
+try:
+  unicode
+except NameError:
+  unicode = str
 
 def build_html_document(body, title=None):
   """Produces the HTML document wrapper for a text/html response."""
@@ -73,7 +79,7 @@ def respond(self, code, headers, body=None):
       self.send_header(key, value)
     self.end_headers()
     if body:
-      if isinstance(body, str):
+      if isinstance(body, unicode):
         body = body.encode('utf-8')
       self.wfile.write(body)