Use ImageFile.MAXBLOCK when saving TIFF images

[radarhere]

Addresses an 'undone' that has been present since #98

Pillow/src/PIL/TiffImagePlugin.py

Lines 1940 to 1941 in 08b561e

	# undone, change to self.decodermaxblock:
	errcode, data = encoder.encode(16 * 1024)[1:]

self.decodermaxblock isn't necessarily available, because im may not be an ImageFile instance, but ImageFile.MAXBLOCK is usually the same.

Pillow/src/PIL/ImageFile.py

Line 128 in 08b561e

self.decodermaxblock = MAXBLOCK