Not adding colon if the imageOrIcon starts with http

Fixing regression issue due to #4

Author: ashwanthkumar

src/main/java/in/ashwanthkumar/slack/webhook/Slack.java

@@ -2,6 +2,7 @@
 
 import in.ashwanthkumar.slack.webhook.service.SlackService;
 import in.ashwanthkumar.utils.collections.Lists;
+import in.ashwanthkumar.utils.lang.StringUtils;
 
 import java.io.IOException;
 import java.net.InetSocketAddress;
@@ -82,7 +83,7 @@ public Slack displayName(String user) {
      * @param imageOrIcon Icon Image URL or emoji code from http://www.emoji-cheat-sheet.com/
      */
     public Slack icon(String imageOrIcon) {
-        if (imageOrIcon != null && !imageOrIcon.isEmpty()) {
+        if (StringUtils.isNotEmpty(imageOrIcon) && !StringUtils.startsWith(imageOrIcon, "http")) {
             if (!imageOrIcon.startsWith(":")) {
                 imageOrIcon = ":" + imageOrIcon;
             }

src/test/java/in/ashwanthkumar/slack/webhook/SlackTest.java

@@ -62,4 +62,15 @@ public void shouldNotAddColonToIconNameIfAlreadyExists() throws IOException {
         assertThat(iconCaptor.getValue(), is(":ghost:"));
     }
 
+    @Test
+    public void shouldNotAddColonToIconNameOrImageIfUrl() throws IOException {
+        Slack slack = new Slack("mockUrl", slackService);
+        slack.icon("http://github.com/i/am/sorry.png");
+        ArgumentCaptor<String> iconCaptor = ArgumentCaptor.forClass(String.class);
+        SlackMessage message = new SlackMessage("hello");
+        doNothing().when(slackService).push(anyString(), eq(message), anyString(), iconCaptor.capture(), anyString(), anyString());
+        slack.sendToChannel("test-channel").push(message);
+        assertThat(iconCaptor.getValue(), is("http://github.com/i/am/sorry.png"));
+    }
+
 }