[fix] TouchableHighlight inactive state

TouchableHighlight didn't preserve its original 'backgroundColor' after
pressOut. This was caused by the inactive background style (transparent)
being applied as an inline style, and so not merging with the original
prop style. The patch sets inactive 'backgroundColor' to 'null' so as to
remove the inline style and render the backgroundColor from props.

Fix #98

src/components/Touchable/TouchableHighlight.js

@@ -32,7 +32,7 @@ type Event = Object;
 
 var DEFAULT_PROPS = {
   activeOpacity: 0.8,
-  underlayColor: 'black',
+  underlayColor: 'black'
 };
 
 var PRESS_RETENTION_OFFSET = {top: 20, left: 20, right: 20, bottom: 30};
@@ -234,7 +234,7 @@ var TouchableHighlight = React.createClass({
         accessibilityLabel={this.props.accessibilityLabel}
         accessibilityRole={this.props.accessibilityRole || this.props.accessibilityTraits || 'button'}
         ref={UNDERLAY_REF}
-        style={[styles.root, this.state.underlayStyle, this.props.style]}
+        style={[styles.root, this.props.style]}
         onLayout={this.props.onLayout}
         hitSlop={this.props.hitSlop}
         onKeyDown={(e) => { this._onKeyEnter(e, this.touchableHandleActivePressIn) }}