Remove the fauly colon from from the Content-Type header

It was throwing an error for some versions of Java. I don't know which or how, but at least it's correct now.

Author: Sam Wierema

api/src/main/java/com/messagebird/MessageBirdServiceImpl.java

@@ -186,11 +186,11 @@ public <P> HttpURLConnection getConnection(final String serviceUrl, final P post
             // could have just used rquestType as it is
             connection.setDoOutput(false);
             connection.setRequestMethod("DELETE");
-            connection.setRequestProperty("Content-Type:", "application/text");
+            connection.setRequestProperty("Content-Type", "application/text");
         } else {
             connection.setDoOutput(false);
             connection.setRequestMethod("GET");
-            connection.setRequestProperty("Content-Type:", "application/text");
+            connection.setRequestProperty("Content-Type", "application/text");
         }
 
         return connection;