Add setUseragent (closes snowplow#87)

Author: jbeemster

src/main/java/com/snowplowanalytics/snowplow/tracker/Subject.java

@@ -102,6 +102,16 @@ public void setIpAddress(String ipAddress) {
         this.standardPairs.put(Parameter.IP_ADDRESS, ipAddress);
     }
 
+    /**
+     * User inputted useragent for the
+     * subject.
+     *
+     * @param useragent a useragent
+     */
+    public void setUseragent(String useragent) {
+        this.standardPairs.put(Parameter.USERAGENT, useragent);
+    }
+
     /**
      * Gets the Subject pairs.
      *

src/main/java/com/snowplowanalytics/snowplow/tracker/constants/Parameter.java

@@ -38,6 +38,7 @@ public class Parameter {
     public static final String TIMEZONE = "tz";
     public static final String LANGUAGE = "lang";
     public static final String IP_ADDRESS = "ip";
+    public static final String USERAGENT = "ua";
 
     // Page View
     public static final String PAGE_URL = "url";

src/test/java/com/snowplowanalytics/snowplow/tracker/SubjectTest.java

@@ -71,6 +71,13 @@ public void testSetIpAddress() throws Exception {
         assertEquals("127.0.0.1", subject.getSubject().get("ip"));
     }
 
+    @Test
+    public void testSetUseragent() throws Exception {
+        Subject subject = new Subject();
+        subject.setUseragent("useragent");
+        assertEquals("useragent", subject.getSubject().get("ua"));
+    }
+
     @Test
     public void testGetSubject() throws Exception {
         Subject subject = new Subject();