fix: Set autocomplete="off" on input to remove SecurityError in Firefox (react-dropzone#476)

Firefox throws a SecurityError if user agent tries to set the input element's value programmatically. To prevent this, `autocomplete="off"` is added to the input attributes.

Fixes react-dropzone#475

Author: dominic-p

src/__snapshots__/index.spec.js.snap

@@ -1,7 +1,7 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`Dropzone basics should render children 1`] = `"<div class=\\"\\" style=\\"width: 200px; height: 200px; border-width: 2px; border-color: #666; border-style: dashed; border-radius: 5px;\\"><p>some content</p><input type=\\"file\\" multiple=\\"\\" style=\\"display: none;\\"></div>"`;
+exports[`Dropzone basics should render children 1`] = `"<div class=\\"\\" style=\\"width: 200px; height: 200px; border-width: 2px; border-color: #666; border-style: dashed; border-radius: 5px;\\"><p>some content</p><input type=\\"file\\" multiple=\\"\\" autocomplete=\\"off\\" style=\\"display: none;\\"></div>"`;
 
-exports[`Dropzone document drop protection does not prevent stray drops when preventDropOnDocument is false 1`] = `"<div class=\\"\\" style=\\"width: 200px; height: 200px; border-width: 2px; border-color: #666; border-style: dashed; border-radius: 5px;\\"><input type=\\"file\\" multiple=\\"\\" style=\\"display: none;\\"></div>"`;
+exports[`Dropzone document drop protection does not prevent stray drops when preventDropOnDocument is false 1`] = `"<div class=\\"\\" style=\\"width: 200px; height: 200px; border-width: 2px; border-color: #666; border-style: dashed; border-radius: 5px;\\"><input type=\\"file\\" multiple=\\"\\" autocomplete=\\"off\\" style=\\"display: none;\\"></div>"`;
 
-exports[`Dropzone document drop protection installs hooks to prevent stray drops from taking over the browser window 1`] = `"<div class=\\"\\" style=\\"width: 200px; height: 200px; border-width: 2px; border-color: #666; border-style: dashed; border-radius: 5px;\\"><p>Content</p><input type=\\"file\\" multiple=\\"\\" style=\\"display: none;\\"></div>"`;
+exports[`Dropzone document drop protection installs hooks to prevent stray drops from taking over the browser window 1`] = `"<div class=\\"\\" style=\\"width: 200px; height: 200px; border-width: 2px; border-color: #666; border-style: dashed; border-radius: 5px;\\"><p>Content</p><input type=\\"file\\" multiple=\\"\\" autocomplete=\\"off\\" style=\\"display: none;\\"></div>"`;

src/index.js

@@ -358,7 +358,8 @@ class Dropzone extends React.Component {
       style: { display: 'none' },
       multiple: supportMultiple && multiple,
       ref: this.setRefs,
-      onChange: this.onDrop
+      onChange: this.onDrop,
+      autoComplete: 'off'
     }
 
     if (name && name.length) {