Compute host header correctly

Signatures need to include the host header, but the requests library
does not include it in prepared requests by default. Rather, it trusts
that Python's HTTP client will compute and inject it when sending the
request. This forces requests-aws4auth to compute how this header will
look like.

A slight discrepancy between the implementations is that the code in
this library unconditionally skips the port, whereas the request ending
up being sent will include a port if it does not match the URL scheme's
default.

This change adjusts the implementations to match in that regard.

Fixes #34

requests_aws4auth/aws4auth.py

@@ -615,7 +615,16 @@ def get_canonical_headers(cls, req, include=None):
         # in the signed headers, but Requests doesn't include it in a
         # PreparedRequest
         if 'host' not in headers:
-            headers['host'] = urlparse(str(req.url)).netloc.split(':')[0]
+            purl = urlparse(str(req.url))
+            netloc = purl.netloc
+            # Python's http client only includes the port if it is non-default,
+            # see http.client.HTTPConnection.putrequest. The request URL, on the
+            # other hand, might explicitly include it.
+            if (purl.port is not None
+                and (purl.scheme == 'http' and purl.port == 80 or
+                     purl.scheme == 'https' and purl.port == 443)):
+                netloc = netloc.rsplit(":", 1)[0]
+            headers['host'] = netloc
         # Aggregate for upper/lowercase header name collisions in header names,
         # AMZ requires values of colliding headers be concatenated into a
         # single header with lowercase name.  Although this is not possible with