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.

Inizia in pytest_mpl/plugin.py, nella chiamata a savefig indicata intorno alle righe 419–421, poi segui come savefig_kwargs proviene dal decoratore del marker. Controlla come viene gestito il buffer in memoria e verifica che il formato predefinito sia PNG, mentre un formato fornito dal chiamante rimanga invariato.