fix X-Amz-Credential parsing for V4 policy signature (#6451)

This commit fixes an AWS S3 incompatibility issue.
The AccessKeyID may contain one or more `/` which caused
the server to interpret parts of the AccessKeyID as
other `X-Amz-Credential` parameters (like date, region, ...)

This commit fixes this by allowing 5 or more
`X-Amz-Credential` parameter strings and only interpreting
the last 5.

Fixes #6443

cmd/signature-v4-parser_test.go

@@ -197,6 +197,25 @@ func TestParseCredentialHeader(t *testing.T) {
 				"aws4_request"),
 			expectedErrCode: ErrNone,
 		},
+		// Test Case - 10.
+		// Test case with right inputs -> AccessKey contains `/`. See minio/#6443
+		// "aws4_request" is the valid request version.
+		{
+			inputCredentialStr: generateCredentialStr(
+				"LOCALKEY/DEV/1",
+				sampleTimeStr,
+				"us-west-1",
+				"s3",
+				"aws4_request"),
+			expectedCredentials: generateCredentials(
+				t,
+				"LOCALKEY/DEV/1",
+				sampleTimeStr,
+				"us-west-1",
+				"s3",
+				"aws4_request"),
+			expectedErrCode: ErrNone,
+		},
 	}
 
 	for i, testCase := range testCases {