Preserve user provided http headers in aws.sign_req

Currently while all the headers are signed the headers in the
returned object are missing all the original user provided headers
This means that if you pass the object directly to http.send
amazon will fail the request because the signed data doesn't match
the canonical request. Users can work around it by using object.union
to restore the original headers, but would be nice to avoid that extra
step

Signed-off-by: Peter <c2zwdjnlcg@users.noreply.github.com>

test/cases/testdata/providers-aws/aws-sign_req.yaml

@@ -43,7 +43,8 @@ cases:
           "Authorization": "AWS4-HMAC-SHA256 Credential=MYAWSACCESSKEYGOESHERE/20151228/us-east-1/s3/aws4_request,SignedHeaders=foo;host;x-amz-content-sha256;x-amz-date,Signature=8f1dc7c9b9978356a0d0989fd26a95307f4f8a4aa264d8220647b7097d839952",
           "host": "example.com",
           "x-amz-content-sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
-          "x-amz-date": "20151228T140825Z"
+          "x-amz-date": "20151228T140825Z",
+          "foo": "bar"
         },
         "method": "get",
         "url": "http://example.com"

topdown/providers.go

@@ -172,10 +172,21 @@ func builtinAWSSigV4SignReq(ctx BuiltinContext, operands []*ast.Term, iter func(
 	}
 
 	// Sign the request object's headers, and reconstruct the headers map.
-	authHeader, signedHeadersMap := aws.SignV4(objectToMap(headers), method, theURL, body, service, awsCreds, signingTimestamp)
+	headersMap := objectToMap(headers)
+	authHeader, awsHeadersMap := aws.SignV4(headersMap, method, theURL, body, service, awsCreds, signingTimestamp)
 	signedHeadersObj := ast.NewObject()
+	// Restore original headers
+	for k, v := range headersMap {
+		// objectToMap doesn't support arrays
+		if len(v) == 1 {
+			signedHeadersObj.Insert(ast.StringTerm(k), ast.StringTerm(v[0]))
+		}
+	}
+	// Set authorization header
 	signedHeadersObj.Insert(ast.StringTerm("Authorization"), ast.StringTerm(authHeader))
-	for k, v := range signedHeadersMap {
+
+	// set aws signature headers
+	for k, v := range awsHeadersMap {
 		signedHeadersObj.Insert(ast.StringTerm(k), ast.StringTerm(v))
 	}