Imaging.writeImage ignores the requested PNM format and writes PPM

[lenamonj]

Imaging.writeImage with ImageFormats.PBM, PGM or PAM writes a P6 PPM. PnmImageParser accepts all five PNM formats, but Imaging passed null parameters, so it always used the default writer. The format reached the parser as PARAM_KEY_FORMAT until IMAGING-159 (406f140, 1.0-alpha3) removed the parameter overloads.

Imaging.writeImage now calls a protected writeImageForFormat hook on AbstractImageParser; the base delegates to writeImage(src, os, null) and PnmImageParser overrides it to set the subtype. PNM keeps the transparency based default. Behaviour change: a transparent image requested as PBM, PGM or PPM now gets that format without its alpha instead of falling through to PAM.

The test fails on master with PBM ==> expected: <P4> but was: <P6>; the default Maven goal passes with 1260 tests.

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