Fix genXmlBodyProperties when body type placeholder

[Ksisu]

Solve #640

Minimalist test code with valign:

    const pptx: PptxGenJS = new PptxGenJS();
    pptx.defineSlideMaster({
      objects: [
        {
          placeholder: {
            options: {
              name: 'title',
              type: 'body',
              valign: pptx.AlignV.middle,
              align: pptx.AlignH.center,
              x: 0,
              y: 0,
              w: '100%',
              h: '100%',
            },
          } as any,
        },
      ],
      title: 'testMaster',
    });
    const slide = pptx.addSlide({masterName: 'testMaster'});
    slide.addText('Test Title', {placeholder: 'title'});
    pptx.writeFile({
      fileName: 'test.pptx',
    });

result:

[gitbrent]

Thanks @Ksisu

This issue was fixed via a recent commit and required a bit more code.