add whereNull and whereNotNull function

Author: ShisunXia

README.md

@@ -258,6 +258,47 @@ The package is developed and tested under Elasticsearch ``v6.*``. It should be a
           $q->orWhere(...)->orWhere(...);
       })->get()->toArray()
       ```
+#### whereNull
+* Parameters
+
+   | Name     | Required | Type                    | Default   | Description                                           |
+   |:--------:|:--------:|:-----------------------:|:---------:|:-----------------------------------------------------:|
+   | column   | Y        | ``string``              |           | Column or relation                                    |
+* Output
+   
+   ``self``
+
+* Examples
+   1. Find all users with no name
+      ```php
+      User::es()->whereNull('name')->get()
+      ```
+   2. Find all users with no address
+         ```php
+         // Find the user whose name starts with 'Leo'
+         User::es()->whereNull('Addresses')->get()
+         ```
+
+#### whereNotNull
+* Parameters
+
+   | Name     | Required | Type                    | Default   | Description                                           |
+   |:--------:|:--------:|:-----------------------:|:---------:|:-----------------------------------------------------:|
+   | column   | Y        | ``string``              |           | Column or relation                                    |
+* Output
+   
+   ``self``
+
+* Examples
+   1. Find all users with a name
+      ```php
+      User::es()->whereNotNull('name')->get()
+      ```
+   2. Find all users with at least one address
+         ```php
+         // Find the user whose name starts with 'Leo'
+         User::es()->whereNotNull('Addresses')->get()
+         ```
 
 #### orWhere
 

src/LaravelElasticsearchQueryBuilder.php

@@ -180,7 +180,12 @@ public function where($column, $operator = null, $value = null, $or = false, $bo
 		$this->validateValue($column_bak, $value);
 		switch($operator) {
 			case '=':
-				$this->query['bool'][$or ? 'should' : 'filter'][] = $value !== null ? ['term' => [$column => $value]] : ['missing' => ['field' => $column]];
+				$this->query['bool'][$or ? 'should' : 'filter'][] = $value !== null ? ['term' => [$column => $value]] :
+					['bool' => ['must_not' => [
+						['exists' => [
+							'field' => $column
+						]]
+					]]];
 				break;
 			case '<':
 				$this->query['bool'][$or ? 'should' : 'filter'][] = ['range' => [$column => ['lt' => $value]]];
@@ -229,7 +234,11 @@ public function where($column, $operator = null, $value = null, $or = false, $bo
 	}
 
 	public function whereNull($column) {
+		return $this->where(snake_case($column), '=', null);
+	}
 
+	public function whereNotNull($column) {
+		return $this->where(snake_case($column), '!=', null);
 	}
 
 	/**
@@ -389,10 +398,10 @@ public function whereHasNull($column, $closure = null, $or = false) {
 					'must_not' => [
 						[
 							'nested' => [
-								'path' => strtolower($column),
+								'path' => snake_case($column),
 								'query' => [
 									'exists' => [
-										'field' => strtolower($column)
+										'field' => snake_case($column)
 									]
 								]
 							]