Fix custom className + regression test

Fixes Hacker0x01#153

Author: martijnrusschen

src/date_input.jsx

@@ -86,7 +86,7 @@ var DateInput = React.createClass( {
         onFocus={this.props.onFocus}
         onBlur={this.props.onBlur}
         onChange={this.handleChange}
-        className="datepicker__input"
+        className={this.props.className}
         disabled={this.props.disabled}
         placeholder={this.props.placeholderText} />;
   }

test/date_input_test.js

@@ -21,4 +21,12 @@ describe( "DateInput", function() {
 
     expect( dateInput.disabled ).to.not.equal( null );
   } );
+
+  it( "uses a custom className if provided", function() {
+    var dateInput = TestUtils.renderIntoDocument(
+      <DateInput className="datepicker__custom-input" />
+    );
+
+    expect( dateInput.getDOMNode().className ).to.equal( "datepicker__custom-input" );
+  } );
 } );