Include body parameter for release

Fixes #4

This adds the ability to include the body parameter when creating the
release.

    - name: Checkout code
      uses: actions/checkout@master
    - name: Create Release
      id: create_release
      uses: actions/create-release@master
      with:
        tag_name: ${{ github.ref }}
        release_name: Release ${{ github.ref }}
        body: Release body

This also supports a multiline body:

    - name: Create Release
      id: create_release
      uses: actions/create-release@master
      with:
        tag_name: ${{ github.ref }}
        release_name: Release ${{ github.ref }}
        body: |
          This is a multiline body
          with more than one
          line

Or if you want the contents of a file:

    - name: Read CHANGELOG
      id: changelog
      run: |
        echo "::set-output name=body::$(cat CHANGELOG.md)"
    - name: Create Release
      id: create_release
      uses: actions/create-release@master
      with:
        release_name: Release ${{ github.ref }}
        body: ${{ steps.changelog.outputs.body }}

One decision I made in favor of less code was to send an empty body when
there was non present. If it would be preferred to send nothing in the
request for the `body` attribute I could instead compose the parameters
with an optional body attribute:

    releaseParams = {
      owner,
      repo,
      tag_name: tag,
      name: releaseName,
      draft,
      prerelease
    };

    if (body) releaseParams.body = body;

    const createReleaseResponse = await github.repos.createRelease( releaseParams );

tests/create-release.test.js

@@ -37,6 +37,7 @@ describe('Create Release', () => {
       .fn()
       .mockReturnValueOnce('refs/tags/v1.0.0')
       .mockReturnValueOnce('myRelease')
+      .mockReturnValueOnce('myBody')
       .mockReturnValueOnce('false')
       .mockReturnValueOnce('false');
 
@@ -47,6 +48,7 @@ describe('Create Release', () => {
       repo: 'repo',
       tag_name: 'v1.0.0',
       name: 'myRelease',
+      body: 'myBody',
       draft: false,
       prerelease: false
     });
@@ -57,6 +59,7 @@ describe('Create Release', () => {
       .fn()
       .mockReturnValueOnce('refs/tags/v1.0.0')
       .mockReturnValueOnce('myRelease')
+      .mockReturnValueOnce('myBody')
       .mockReturnValueOnce('true')
       .mockReturnValueOnce('false');
 
@@ -67,6 +70,7 @@ describe('Create Release', () => {
       repo: 'repo',
       tag_name: 'v1.0.0',
       name: 'myRelease',
+      body: 'myBody',
       draft: true,
       prerelease: false
     });
@@ -77,6 +81,7 @@ describe('Create Release', () => {
       .fn()
       .mockReturnValueOnce('refs/tags/v1.0.0')
       .mockReturnValueOnce('myRelease')
+      .mockReturnValueOnce('myBody')
       .mockReturnValueOnce('false')
       .mockReturnValueOnce('true');
 
@@ -87,16 +92,40 @@ describe('Create Release', () => {
       repo: 'repo',
       tag_name: 'v1.0.0',
       name: 'myRelease',
+      body: 'myBody',
       draft: false,
       prerelease: true
     });
   });
 
+  test('Release with empty body is created', async () => {
+    core.getInput = jest
+      .fn()
+      .mockReturnValueOnce('refs/tags/v1.0.0')
+      .mockReturnValueOnce('myRelease')
+      .mockReturnValueOnce('') // <-- The default value for body in action.yml
+      .mockReturnValueOnce('false')
+      .mockReturnValueOnce('false');
+
+    await run();
+
+    expect(createRelease).toHaveBeenCalledWith({
+      owner: 'owner',
+      repo: 'repo',
+      tag_name: 'v1.0.0',
+      name: 'myRelease',
+      body: '',
+      draft: false,
+      prerelease: false
+    });
+  });
+
   test('Outputs are set', async () => {
     core.getInput = jest
       .fn()
       .mockReturnValueOnce('refs/tags/v1.0.0')
       .mockReturnValueOnce('myRelease')
+      .mockReturnValueOnce('myBody')
       .mockReturnValueOnce('false')
       .mockReturnValueOnce('false');
 
@@ -114,6 +143,7 @@ describe('Create Release', () => {
       .fn()
       .mockReturnValueOnce('refs/tags/v1.0.0')
       .mockReturnValueOnce('myRelease')
+      .mockReturnValueOnce('myBody')
       .mockReturnValueOnce('false')
       .mockReturnValueOnce('false');