Add some tests.

dd-trace/src/test/groovy/com/datadoghq/trace/DDSpanContextTest.groovy

@@ -4,4 +4,54 @@ import spock.lang.Specification
 
 class DDSpanContextTest extends Specification {
 
+  def "null values for tags are ignored"() {
+    setup:
+    def context = SpanFactory.newSpanOf(0).context
+    context.setTag(name, null)
+
+    expect:
+    context.getTags() == [(DDTags.THREAD_NAME): Thread.currentThread().name, (DDTags.THREAD_ID): Thread.currentThread().id]
+    context.serviceName == "fakeService"
+    context.resourceName == "fakeResource"
+    context.spanType == "fakeType"
+
+    where:
+    name << [DDTags.SERVICE_NAME, DDTags.RESOURCE_NAME, DDTags.SPAN_TYPE, "some.tag"]
+  }
+
+  def "special tags set certain values"() {
+    setup:
+    def context = SpanFactory.newSpanOf(0).context
+    context.setTag(name, value)
+
+    expect:
+    context.getTags() == [(DDTags.THREAD_NAME): Thread.currentThread().name, (DDTags.THREAD_ID): Thread.currentThread().id]
+    context."$method" == value
+
+    where:
+    name                 | value                | method
+    DDTags.SERVICE_NAME  | "different service"  | "serviceName"
+    DDTags.RESOURCE_NAME | "different resource" | "resourceName"
+    DDTags.SPAN_TYPE     | "different type"     | "spanType"
+  }
+
+  def "tags can be added to the context"() {
+    setup:
+    def context = SpanFactory.newSpanOf(0).context
+    context.setTag(name, value)
+
+    expect:
+    context.getTags() == [
+      (name)              : value,
+      (DDTags.THREAD_NAME): Thread.currentThread().name,
+      (DDTags.THREAD_ID)  : Thread.currentThread().id
+    ]
+
+    where:
+    name             | value
+    "some.name"      | "some value"
+    "tag with int"   | 1234
+    "tag-with-bool"  | false
+    "tag_with_float" | 0.321
+  }
 }