caucase.http: Fix server_version encoding for py2.7 .

Work around what should be a test-only issue, where versioneer produces
a unicode object for the module's version (by decoding a json object,
whose strings become unicode objects). Python 2.7's BaseHttpServer does
not encode the response in such case, causing a test failure when writing
to wfile, set to be a BytesIO object.
I guess some magic encoding happens on a real socket, likely to ascii.

caucase/http.py

@@ -176,6 +176,17 @@ class CaucaseWSGIRequestHandler(WSGIRequestHandler):
   other than stderr for log output.
   """
   server_version = 'caucased ' + version.__version__
+  # pylint: disable=undefined-variable
+  if sys.version_info[0] < 3 and isinstance(server_version, unicode): # pragma: no cover
+  # pylint: enable=undefined-variable
+    # BBB: py2.7
+    # Unicode objects are expected in py3, but unexpected in py2.
+    # When running on an installed egg, versioneer returns a unicode object for
+    # the version, making server_version the only unicode object sent over http
+    # in this source. Normalise it.
+    # This should be a test-only issue (as tests set a BytesIO object as wfile
+    # in testWSGIBase) but consistency should not hurt in general.
+    server_version = server_version.encode('ascii')
   remote_user_name = '-'
 
   def __init__(self, *args, **kw):