default savefig format

In the process of penning #150, I noticed that when capturing the test image to an io.Bytes in-memory buffer:

https://github.com/matplotlib/pytest-mpl/blob/e3876180847c2d04aeda1982372074998a7f8716/pytest_mpl/plugin.py#L419-L421

That the format kwarg to savefig isn't used.

Would you be adverse to defaulting to format='png', particularly when writing to a buffer, as (from what I understand) matplotlib will default to the rcParams to select the file-format to save to the buffer for the image, and this could be a source confusion/issues for hashing repeatability for developers when testing.

i.e., what I'm proposing is that a format='png' is injected into the savefig_kwargs, iff not specified by the caller as a kwarg option in the marker decorator.

pytest_mpl/plugin.py の 419～421 行付近にあるリンク先の savefig 呼び出しから始め、次に savefig_kwargs が marker デコレーターからどのように渡されるかを追跡します。メモリ内バッファがどのように扱われるかを確認し、デフォルトの形式が PNG であり、呼び出し元が指定した形式は変更されないことを検証します。