Fix policy bugs Null conditions and canonical names (#7021)

This PR fixes two different issues

- Null condition implementation
- HTTP Canonical request value names

This PR fixes handling of null conditions and
handle HTTP canonical names in request values.

This PR was tested with policies mentioned in the following blog
https://aws.amazon.com/blogs/security/how-to-prevent-uploads-of-unencrypted-objects-to-amazon-s3/

Fixes #6955

pkg/policy/condition/stringequalsfunc.go

@@ -18,6 +18,7 @@ package condition
 
 import (
 	"fmt"
+	"net/http"
 	"sort"
 	"strings"
 
@@ -44,7 +45,11 @@ type stringEqualsFunc struct {
 // evaluate() - evaluates to check whether value by Key in given values is in
 // condition values.
 func (f stringEqualsFunc) evaluate(values map[string][]string) bool {
-	requestValue := values[f.k.Name()]
+	requestValue, ok := values[http.CanonicalHeaderKey(f.k.Name())]
+	if !ok {
+		requestValue = values[f.k.Name()]
+	}
+
 	return !f.values.Intersection(set.CreateStringSet(requestValue...)).IsEmpty()
 }