Add subquery test code

[bourbonkk]

Hi, I wrote a test code for subquery. #15 #78

@jbguerraz @saketbairoliya2 @vigith

There were two code modifications.

1. Query struct : "-,omitempty" -> "query,omitempty"

type Query struct {
	Base
	Query builder.Query `json:"query,omitempty"`
}

This is necessary to make a subquery.

2. Lower struct : "locale,omitempty" -> "locale"

type Lower struct {
	Base
	Locale string `json:"locale"`
}

"extractionFn": {
"locale": "",
"type": "lower"
},

To express this example code,

[jbguerraz]

Hello @bourbonkk
Thank you!
I'm just back from vacation, I'll check it out ASAP (probably next week)

[jbguerraz]

Mostly there :) but we still have an issue with loading query (with subquery) from json

[saketbairoliya2]

@bourbonkk @jbguerraz - Any updates on this PR?

[bourbonkk]

@saketbairoliya2 No, it doesn't.

[saketbairoliya2]

@bourbonkk - So are we planning to merge this one or what's the further plan?

[bourbonkk]

This PR has only reviews left.
@saketbairoliya2 @jbguerraz

[saketbairoliya2]

Okay - May be @jbguerraz would be a better person to approve this PR. Waiting for him to get back. Thanks @bourbonkk