fix #134 - include base_ref and head_ref in github context from the event json

act/runner/testdata/pull-request/event.json

@@ -0,0 +1,10 @@
+{
+  "pull_request": {
+    "head": {
+      "ref": "sample-head-ref"
+    },
+    "base": {
+      "ref": "sample-base-ref"
+    }
+  }
+}

act/runner/testdata/pull-request/main.yaml

@@ -0,0 +1,10 @@
+name: basic
+on: pull_request
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo '${{github.ref}}' 
+      - run: echo '${{github.head_ref}}' | grep sample-head-ref
+      - run: echo '${{github.base_ref}}' | grep sample-base-ref