Migrate to go1.12 to simplify our cmd/http package (#7302)

Simplify the cmd/http package overall by removing
custom plain text v/s tls connection detection, by
migrating to go1.12 and choose minimum version
to be go1.12

Also remove all the vendored deps, since they
are not useful anymore.

main_test.go

@@ -1,46 +0,0 @@
-/*
- * Minio Cloud Storage, (C) 2017, 2018 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 TestCheckGoVersion(t *testing.T) {
-	// Test success cases.
-	testCases := []struct {
-		version string
-		success bool
-	}{
-		{minGoVersion, true},
-		{"1.6.8", false},
-		{"1.5", false},
-		{"0.1", false},
-		{".1", false},
-		{"somejunk", false},
-	}
-
-	for _, testCase := range testCases {
-		err := checkGoVersion(testCase.version)
-		if err != nil && testCase.success {
-			t.Fatalf("Test %v, expected: success, got: %v", testCase, err)
-		}
-		if err == nil && !testCase.success {
-			t.Fatalf("Test %v, expected: failure, got: success", testCase)
-		}
-	}
-}