TST: fix TypeError: string argument expected, got 'bytes' on Py3

Fixes two test errors on Python 3:
```
Tests\test_file_webp_metadata.py:45: image.save(buffer, "webp", exif=expected_exif) failed:
Traceback (most recent call last):
  File "Tests\test_file_webp_metadata.py", line 45, in test_write_exif_metadata
    image.save(buffer, "webp", exif=expected_exif)
  File "PIL\Image.py", line 1441, in save
    save_handler(self, fp, filename)
  File "PIL\WebPImagePlugin.py", line 70, in _save
    fp.write(data)
TypeError: string argument expected, got 'bytes'
```
```
Tests\test_file_webp_metadata.py:78: image.save(buffer, "webp", icc_profile=expected_icc_profile) failed:
Traceback (most recent call last):
  File "Tests\test_file_webp_metadata.py", line 78, in test_write_icc_metadata
    image.save(buffer, "webp", icc_profile=expected_icc_profile)
  File "PIL\Image.py", line 1441, in save
    save_handler(self, fp, filename)
  File "PIL\WebPImagePlugin.py", line 70, in _save
    fp.write(data)
TypeError: string argument expected, got 'bytes'
```