Default background color in case of png is transparent

Author: MASNathan

README.md

@@ -65,6 +65,7 @@ And the resulted image format
 // allowed formats are 'jpg' and 'png', default is 'jpg'.
 $screenCapture->setFormat('png');
 ```
+* If the format is ```jpg``` and the background color is not set, the default value will be ```#FFFFFF```, if ```png``` the default background color will be transparent.
 
 And most importantly, save the result
 ``` php

demo/index.html

@@ -97,7 +97,6 @@ <h2>Give it a test drive...</h2>
         <input type="number" class="form-control" name="h" placeholder="768">
       </div>
 
-
       <div class="form-group col-md-6">
         <label for="user-agent">User Agent String</label>
         <input type="text" class="form-control" name="user-agent"
@@ -115,7 +114,7 @@ <h2>Give it a test drive...</h2>
       <div class="col-md-6"></div>
       <div class="form-group col-md-3">
         <label for="bg-color">Background color</label>
-        <input type="text" name="bg-color" value="#ffffff" class="form-control color-picker" data-format="hex"/>
+        <input type="text" name="bg-color" value="" class="form-control color-picker" data-format="hex"/>
       </div>
 
       <div class="form-group col-md-3">

src/Capture.php

@@ -54,7 +54,7 @@ class Capture
      *
      * @var string
      */
-    protected $backgroundColor = '#FFFFFF';
+    protected $backgroundColor = '';
 
     /**
      * Image format
@@ -135,6 +135,10 @@ public function save($imageLocation, $deleteFileIfExists = true)
 
         if ($this->backgroundColor) {
             $data['backgroundColor'] = $this->backgroundColor;
+        } elseif ($this->getFormat() == 'jpg') {
+            // If there is no background color set, and it's a jpeg
+            // we need to set a bg color, otherwise the background will be black
+            $data['backgroundColor'] = '#FFFFFF';
         }
 
         if ($this->userAgentString) {