bug: echo plugin can not set headers

[idbeta]

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The headers option will automatically refresh as Opthion3 when setting the other option,
and the headers option can not be set.

[liuxiran]

There are two errors here:

1. headers schema should be transformed just like proxy-rewrite and response-rewrite in fe plugin component

2. echo schema still uses Draft5, that why we will see select element Option1 in page. so It also needs to change the echo shema to Draft7.

{
    "additionalProperties": false,
    "required": ["before_body","body","after_body"],
    "minProperties": 1,
    "properties": {
	"after_body": {
		"description": "body after the modification of filter phase.",
			"type": "string"
		},
		"auth_value": {
			"description": "auth value",
			"type": "string"
		},
		"before_body": {
			"description": "body before the filter phase.",
			"type": "string"
		},
		"body": {
			"description": "body to replace upstream response.",
			"type": "string"
		},
		"headers": {
			"description": "new headers for response",
			"minProperties":1,
			"type": "object"
		}
	},
	"type": "object"
}

[nic-chen]

fixed by code-mirror

[juzhiyuan]

closed by #898