api: DRY code and add new test

This commit makes code cleaner and reduces the repetitions in the code
base. Specifically, it reduces the clutter in setObjectHeaders. It also
merges encodeSuccessResponse and encodeErrorResponse together because
they served no purpose differently. Finally, it adds a simple test for
generateRequestID.

api-headers_test.go

@@ -0,0 +1,40 @@
+/*
+ * Minio Cloud Storage, (C) 2015, 2016 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package main
+
+import (
+	"testing"
+)
+
+func TestGenerateRequestID(t *testing.T) {
+	// Ensure that it returns an alphanumeric result of length 16.
+	var id []byte = generateRequestID()
+
+	if len(id) != 16 {
+		t.Fail()
+	}
+
+	var e rune
+	for _, char := range id {
+		e = rune(char)
+
+		// Ensure that it is alphanumeric, in this case, between 0-9 and A-Z.
+		if !(('0' <= e && e <= '9') || ('A' <= e && e <= 'Z')) {
+			t.Fail()
+		}
+	}
+}