updated docs with new config

Author: Dipak Sarkar

docs/guide/config.md

@@ -26,7 +26,7 @@ The following options can be passed as an object literal to the NumberFormat fun
 - Type: `number`
 - Default: `2`
 - Required: `false`
-## null_value
+## nullValue
 
 - Type: `number`
 - Default: `0`

tests/unit/number-format.custom.spec.js

@@ -5,15 +5,15 @@ describe('should not throw error on empty config', () => {
     prefix: '$',
     separator: '.',
     decimal: ',',
-    null_value: 0,
+    nullValue: 0,
   })).not.toThrow()
 })
 describe('when the value is invalid with custom config', () => {
   const numberFormat = new NumberFormat({
     prefix: '$',
     separator: '.',
     decimal: ',',
-    null_value: 0,
+    nullValue: 0,
   })
   it('should return as follows', () => {
     expect(numberFormat.format('')).toEqual('')
@@ -44,7 +44,7 @@ describe('format when options are custom', () => {
     prefix: '$',
     separator: '.',
     decimal: ',',
-    null_value: '',
+    nullValue: '',
   })
   it('format string value', () => {
     expect(numberFormat.format('0')).toEqual('$0')
@@ -88,7 +88,7 @@ describe('unformat when options are default', () => {
     prefix: '$',
     separator: '.',
     decimal: ',',
-    null_value: '',
+    nullValue: '',
   })
   it('unformat string value', () => {
     expect(numberFormat.clean(true).unformat('0')).toEqual('0')