cleanup: All conditionals simplified under pkg. (#3875)

Address all the changes reported/recommended by
`gosimple` tool.

pkg/mimedb/db_test.go

@@ -1,5 +1,5 @@
 /*
- * mime-db: Mime Database, (C) 2015, 2016 Minio, Inc.
+ * mime-db: Mime Database, (C) 2015, 2016, 2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,7 @@ func TestMimeLookup(t *testing.T) {
 		t.Fatalf("Invalid content type are found expected \"application/x-msdownload\", got %s", contentType)
 	}
 	compressible := DB["txt"].Compressible
-	if compressible != false {
+	if compressible {
 		t.Fatalf("Invalid content type are found expected \"false\", got %t", compressible)
 	}
 }